Handles the RIL error from the CommandException

In RadioResponse.java, all the RIL errors will be converted into
CommandException and sent back to the caller with the callback message.
So the AsyncResult in the callback message of 'EVENT_START_NETWORK_SCAN_DONE'
and 'EVENT_START_NETWORK_SCAN_DONE' will either contains a NetworkScanResult
without any exception, or contains an exception without any NetworkScanResult.
So if there is any error when trying to start/stop a scan, the error
code must be extracted from the CommandException.

Cherry-picked cleanly from:
https://android-review.git.corp.google.com/#/c/415900/

Test: Telephony sanity tests
Merged-in: I9766b7fd530e9146e6d9867478ed79ad8f7ba1fc
Change-Id: I9766b7fd530e9146e6d9867478ed79ad8f7ba1fc
(cherry picked from commit a234d03c)