gdb/testsuite/ada: Handle missing debug info case

Update a test script to handle the case where missing Ada debug
information means we can't catch exceptions.  This was discussed on
the list here:

  https://sourceware.org/ml/gdb-patches/2019-08/msg00607.html

And is similar to code that already exists in the test scripts
gdb.ada/catch_ex.exp and gdb.ada/mi_catch_ex.exp.

gdb/testsuite/ChangeLog:

	* gdb.ada/catch_ex_std.exp: Handle being unabled to catch Ada
	exceptions due to missing debug information.