pk-client: Handle few error cases to avoid crash

[sidt4]

Without #674:

$ pkcon -p get-details /etc/passwd
Transaction:	Getting details
Status: 	Waiting in queue
The daemon crashed mid-transaction!

With #674:

$ pkcon -p get-details /etc/passwd
Transaction:	Getting details
Status: 	Waiting in queue
Status: 	Finished

(pkcon:426791): GLib-GIO-CRITICAL **: 21:26:00.688: g_simple_async_result_set_from_error: assertion 'error != NULL' failed
Trace/breakpoint trap (core dumped)

With #674 and this MR:

$ pkcon -p get-details /etc/passwd
Transaction:	Getting details
Status: 	Waiting in queue
Status: 	Finished
Results:
Fatal error: File /etc/passwd is not found or unsupported