[gdb/testsuite] Improve quotemeta error messages

[gdb/testsuite] Improve quotemeta error messages

With test-case gdb.testsuite/quotemeta.exp we get:
...
check_quotemeta_fail: input: @NOTHING
check_quotemeta_fail: catch_res: 1, catch_msg: \
  can't read "::_quotemeta(NOTHING)": no such element in array
PASS: gdb.testsuite/quotemeta.exp: no-such-subst
check_quotemeta_fail: input: @bad-name
check_quotemeta_fail: catch_res: 1, catch_msg: invalid quotemeta expression
PASS: gdb.testsuite/quotemeta.exp: bad-name
...

Improve the error messages to:
- undefined quotemeta expression @NOTHING
- invalid quotemeta expression @bad-name