network_3GDisableWhileConnecting: Report if no device/service is found

network_3GDisableWhileConnecting previously triggered the following
Python exception when the test could not find a cellular device on the
DUT:

  AttributeError: 'NoneType' object has no attribute 'Disable'

This CL changes network_3GDisableWhileConnecting to properly raise an
test error when no cellular device or service is found.

BUG=None
TEST=Run network_3GDisableWhileConnecting test.

Change-Id: Ib6afa05228e8b11a0a5e7602442e3bb8cf47779a
Reviewed-on: https://chromium-review.googlesource.com/204892
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
1 file changed