Use SCOPE_EXIT in write_gcore_file

Use SCOPE_EXIT in write_gcore_file

This replaces a TRY/CATCH in write_gcore_file with a use of SCOPE_EXIT
instead.  I find that this is simpler to understand.

2019-03-06  Tom Tromey  <tom@tromey.com>

	* gcore.c (write_gcore_file): Use SCOPE_EXIT.