Avoid FAILs in gdb.compile

Avoid FAILs in gdb.compile

Many gdb.compile C++ tests fail for me on Fedora 36.  I think these
are largely bugs in the plugin, though I didn't investigate too
deeply.  Once one failure is seen, this often cascades and sometimes
there are many timeouts.

For example, this can happen:

    (gdb) compile code var = a->get_var ()
    warning: Could not find symbol "_ZZ9_gdb_exprP10__gdb_regsE1a" for compiled module "/tmp/gdbobj-0xdI6U/out2.o".
    1 symbols were missing, cannot continue.

I think this is probably a plugin bug because, IIRC, in theory these
symbols should be exempt from a lookup via gdb.

This patch arranges to catch any catastrophic failure and then simply
exit the entire .exp file.