Consolidate and standardize error handling throughout
lwm2m_obj_firmware_pull.c. As well as handle previously
unhandled errors returned from transfer_request().
NOTE: in general, unhandled errors will now result in
RESULT_UPDATE_FAILED. Previously, unhandled errors in
transfer_request() would result in RESULT_CONNECTION_LOST
which might or might not be over-written with another
result later.
Signed-off-by: Michael Scott <michael@opensourcefoundries.com>