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/MercuryFatalError.java

@@ -0,0 +1,23 @@
+// vim: ts=4 sw=4 expandtab ft=java
+//
+// Copyright (C) 2024 The Mercury Team.
+// This file is distributed under the terms specified in COPYING.LIB.
+//
+// This exception is for when fatal errors in the Mercury runtime.
+
+package jmercury.runtime;
+
+public class MercuryFatalError extends java.lang.Error {
+
+    public MercuryFatalError(String message) {
+        super(message);
+    }
+
+    public MercuryFatalError(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public MercuryFatalError(Throwable cause) {
+        super(cause);
+    }
+}