[gdb, c++] Improve error message when using libstdcxx without SDT probes

[gdb, c++] Improve error message when using libstdcxx without SDT probes

When using catch catch/rethrow/catch, a libstdcxx with SDT probes is required
for both the regexp argument, and the convenience variable $_exception (
https://sourceware.org/gdb/current/onlinedocs/gdb/Set-Catchpoints.html ).

Currently, when using these features with a libstdcxx without SDT probes, we
get the cryptic error message:
...
not stopped at a C++ exception catchpoint
...

Improve this by instead emitting the more helpful:
...
did not find exception probe (does libstdcxx have SDT probes?)
...

Tested on x86_64-linux.

gdb/ChangeLog:

2019-08-26  Tom de Vries  <tdevries@suse.de>

	PR c++/24852
	* break-catch-throw.c (fetch_probe_arguments): Improve error mesage
	when pc_probe.prob == NULL.

gdb/testsuite/ChangeLog:

2019-08-26  Tom de Vries  <tdevries@suse.de>

	PR c++/24852
	* gdb.cp/no-libstdcxx-probe.exp: New test.