Fix exception handling when messages are null

[ssb22]

This is a redo of #21 against the modern repository. It changes the code to avoid relying on getMessage() when reporting exceptions, since getMessage() can return null and users find "null" in error messages to be uninformative as shown by issue #18.

The default toString() method itself calls getMessage() and, if it's null, at least returns the class name of the exception, which is better than just "null". Moreover, toString() is called implicitly if we pass a non-String object to println or add it to a string.

While I was at it, I changed a potential null dereference in an exception handler (use "Help".equals(message) not message.equals("Help") because the latter could crash if message is null), and I also cleared the "executable" bit from the permissions of obviously non-executable source files.