LastFm: Avoid crashing if scrobbling response is not JSON (bgo#689016)

Improves exception handling for invalid responses from Last.fm. The
specific reported in bgo#689016 is about a '414' error from Last.fm but
this change would catch any exception thrown while processing the last.fm
response.

This only addresses the crash caused by not handling the exception,
not the root cause.

Signed-off-by: Andres G. Aragoneses <knocte@gmail.com>