[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?)
...