Cli error message improvements (#172)

* Errors in Generate/Validate print to stderr/exit 1

Generate and Validate exposed exceptions rather than user-friendly
messages when an error occurred. In generate, this could happen for
numerous reasons, but the most likely is a user typing (or guessing) an
invalid generator name. In Validate, an error was exposed if there were
any validation errors in a spec.

New behavior:

* Generate now exposes a typed exception when a generator cannot be
  loaded by name. This allows consistent messaging for load failures.
* Generate now presents guidance on failure (check the spelling and try
  again). This is purely a usability improvement.
* Validate now writes validation errors to stderr and exits with code 1.

* Improve err messages: config-help/required opts.

config-help now presents same error for invalid generator names as the
'generate' command.

Options which are required, and those which require a value, now present
a user-friendly hint at the error and exit with code 1 (rather than an
uncaught exception).

* Log missing -g error to stderr rather than LOGGER

modules/openapi-generator/src/main/java/org/openapitools/codegen/CodegenConfigLoader.java

@@ -47,7 +47,7 @@ public class CodegenConfigLoader {
         try {
             return (CodegenConfig) Class.forName(name).newInstance();
         } catch (Exception e) {
-            throw new RuntimeException("Can't load config class with name '".concat(name) + "'\nAvailable:\n" + availableConfigs.toString());
+            throw new GeneratorNotFoundException("Can't load config class with name '".concat(name) + "'\nAvailable:\n" + availableConfigs.toString(), e);
         }
     }
 

modules/openapi-generator/src/main/java/org/openapitools/codegen/GeneratorNotFoundException.java

@@ -0,0 +1,80 @@
+package org.openapitools.codegen;
+
+/**
+ * Typed exception exposing issues with loading generators (e.g. by name).
+ */
+@SuppressWarnings("unused")
+public class GeneratorNotFoundException extends RuntimeException {
+    /**
+     * Constructs a new runtime exception with {@code null} as its
+     * detail message.  The cause is not initialized, and may subsequently be
+     * initialized by a call to {@link #initCause}.
+     */
+    public GeneratorNotFoundException() {
+    }
+
+    /**
+     * Constructs a new runtime exception with the specified detail message.
+     * The cause is not initialized, and may subsequently be initialized by a
+     * call to {@link #initCause}.
+     *
+     * @param message the detail message. The detail message is saved for
+     *                later retrieval by the {@link #getMessage()} method.
+     */
+    public GeneratorNotFoundException(String message) {
+        super(message);
+    }
+
+    /**
+     * Constructs a new runtime exception with the specified detail message and
+     * cause.  <p>Note that the detail message associated with
+     * {@code cause} is <i>not</i> automatically incorporated in
+     * this runtime exception's detail message.
+     *
+     * @param message the detail message (which is saved for later retrieval
+     *                by the {@link #getMessage()} method).
+     * @param cause   the cause (which is saved for later retrieval by the
+     *                {@link #getCause()} method).  (A <tt>null</tt> value is
+     *                permitted, and indicates that the cause is nonexistent or
+     *                unknown.)
+     * @since 1.4
+     */
+    public GeneratorNotFoundException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    /**
+     * Constructs a new runtime exception with the specified cause and a
+     * detail message of <tt>(cause==null ? null : cause.toString())</tt>
+     * (which typically contains the class and detail message of
+     * <tt>cause</tt>).  This constructor is useful for runtime exceptions
+     * that are little more than wrappers for other throwables.
+     *
+     * @param cause the cause (which is saved for later retrieval by the
+     *              {@link #getCause()} method).  (A <tt>null</tt> value is
+     *              permitted, and indicates that the cause is nonexistent or
+     *              unknown.)
+     * @since 1.4
+     */
+    public GeneratorNotFoundException(Throwable cause) {
+        super(cause);
+    }
+
+    /**
+     * Constructs a new runtime exception with the specified detail
+     * message, cause, suppression enabled or disabled, and writable
+     * stack trace enabled or disabled.
+     *
+     * @param message            the detail message.
+     * @param cause              the cause.  (A {@code null} value is permitted,
+     *                           and indicates that the cause is nonexistent or unknown.)
+     * @param enableSuppression  whether or not suppression is enabled
+     *                           or disabled
+     * @param writableStackTrace whether or not the stack trace should
+     *                           be writable
+     * @since 1.7
+     */
+    public GeneratorNotFoundException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
+        super(message, cause, enableSuppression, writableStackTrace);
+    }
+}