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/MercuryWorkerThread.java

@@ -61,7 +61,8 @@ public class MercuryWorkerThread extends MercuryThread
                         // Make the thread exit after throwing an exception.
                         break;
                     } catch (Throwable e) {
-                        // Some other error occured. bail out.
+                        // Some other error occured, bail out.
+                        // XXX We should avoid calling exit() here.
                         System.err.println("Uncaught exception: " +
                             e.toString());
                         System.err.println(e.getMessage());