Throw and catch exceptions for less bug prone lambda termination

Previously, the initial comparison for mApfFilter == null called
completeExceptionally() without actually breaking out of the lambda.

In order to prevent this bug in the future, it is better to actually
throw and catch the exception.

This also slightly rewords an exception message to make it fit on one
line.

Test: adb shell cmd network_stack apf wlan0 status
Change-Id: Ia7502cc74b3a0498fc4a14d9560acb60c6286cf9
1 file changed