Allow try_catch.h to be used with other projects

I wanted to be able to share the MRI defined exception codes with my
pinkySim code.  This change allows that to happen.  The pinkySim code
can include MRI's try_catch.h after defining the
MRI_SKIP_TRY_CATCH_MACRO_DEFINES macro to only pull in the
__mriExceptionCode extern definition and the exception codes.  The
pinkySim try_catch.h can then add its own exception codes starting at
mriMaxException and still use its longjmp version of try/catch macros
instead.