Add an onError() method to CellInfoCallback

There are 2 known types of failures that result in
CellInfo requests to fail:
1) timeout waiting for a response from the modem
2) errors returned from the modem (including the RIL).

This patch adds an explicit onError() callback to
ensure that callers can get detailed error information.

Bug: 120488917
Test: manually tested both error cases by modifying
    RadioResponse.java and forcing both the timeout
    and an explicit error.
Change-Id: Ieb499dc220718aa99a552ea081eb4b4b5c1424aa