Warn if missing debug info for Ada exception catchpoints

Warn if missing debug info for Ada exception catchpoints

This patch should help the user understand why the debugger is not
able to insert Ada exception catchpoints when the Ada runtime was
stripped of debugging info, as is often the case on many GNU/Linux
distros:

    (gdb) catch exception
    Your Ada runtime appears to be missing some debugging information.
    Cannot insert Ada exception catchpoint in this configuration.

gdb/ChangeLog:

        * ada-lang.c (ada_has_this_exception_support): Raise an error
        if we could find the Ada exception hook in the Ada runtime,
        but no debugging info for that hook.

gdb/testsuite/ChangeLog:

        * gdb.ada/catch_ex.exp, gdb.ada/mi_catch_ex.exp: Adjust
        expected output for unsupported case.