Add option to symbol import script to ignore errors in input

Summary:
I was running the generate_ctags_symbols script on a project, and some
combination of this script and ctags was generating invalid rows for this
script. It's nice if this script can ignore invalid input instead of having
to add another step to filter out invalid input.

Test Plan: run script with input that has some invalid lines

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3532