Add MercuryFatalError exception to the Java runtime.

Add MercuryFatalError, a new Java exception that is intended to be used for a
similar purpose to the C runtime's MR_fatal_error() function.

Throw a MercuryFatalError exception instead of calling System.exit() in a spot.
Calling exit() is fine for executables, but for code that is deployed in an
application server calling exit() may shut down the entire server.

java/runtime/MercuryFatalError.java:
    Add the new exception.

java/runtime/MercuryOptions.java:
    Do not call System.exit() when we encounter an unrecognized
    option in MERCURY_OPTIONS, throw a MercuryFatalError exception
    instead.

    Refactor the process() method in order to avoid indentation.

    Catch NumberFormatExceptions thrown when attempting to convert
    integer option values; rethrow them as MercuryFatalErrors.
    (XXX the C version of the runtime just ignores this error.)

java/runtime/JavaInternal.java:
    Catch and report MercuryFatalErrors in the runMain() method.

java/runtime/MercuryWorkerThread.java:
    Add an XXX about a call to System.exit() here.

java/runtime/JavaInternal.java

@@ -77,7 +77,17 @@ public class JavaInternal {
      */
     public static void runMain(Runnable main)
     {
-        getThreadPool().runMain(main);
+        try {
+            getThreadPool().runMain(main);
+        } catch (jmercury.runtime.MercuryFatalError e) {
+            System.out.flush();
+            System.err.println("Mercury runtime: " + e.getMessage());
+            System.err.flush();
+
+            if (exit_status == 0) {
+                exit_status = 1;
+            }
+        }
     }
 
     /**