Fix segfaults in some error cases.

BEFORE:
$ aidl -h
aidl: option requires an argument -- 'h'
Segmentation fault
$ aidl asdf
terminating with uncaught exception of type std::out_of_range: basic_string
Aborted

NOW:
$ aidl -h
aidl: option requires an argument -- 'h'
usage:
...
$ aidl adsf
Expected .aidl file for input but got 'adsf'

(also, some error checks were missing)

Bug: N/A
Test: runtests.sh
Change-Id: I28584fdf3eb74a64cec260e80fc32f6c9cd99f4e
1 file changed