[ModemManager] [PATCH] mm_gsm_parse_scan_response(): Improve regex-construction error handling

[Nathan Williams <njw google com>]

This was the only code in ModemManager using g_error(), and not
appropriately, I think.
The codebase is still mixed on how to handle errors compiling regular
expressions; other places in this file use g_assert(), while the rest
of the code tends to report some kind of parse-failure error. Perhaps
this makes sense in terms of startup versus while-running operations?

    - Nathan