Print unhandled exception's trace and abort if rt_trapExceptions is false

D exception handling was assuming the top level exception handler,
so all exceptions would be handled somewhere. In case rt_trapExceptions
is cleared, this handler would not be enabled, and this assertion
would fail, aborting the program, but without any information
about the exception. This is now changed, so the exception information
will be printed to the stderr, followed by abort.
