Handle RIL errors properly

Separate out RemoteException and RuntimeException so the latter throws a
SYSTEM_ERR instead of setting the radio to unavailable.
If the radio proxy does not exist, return REQUEST_NOT_SUPPORTED so the
onComplete message is sent instead of hanging

Test: atest RILTest
Bug: 231996546
Change-Id: I62e0593e6ff338f43b889287143b54137e47599d