Fix bugs in python lexer

Summary:
Two things:

• The appearance of \\ or \' or \-followed-by-a-newline in a raw string would improperly end the string
• A disallowed newline in a string would cause an error instead of trying to continue anyway

Test Plan:
Ran it on a file that was failing for me that was something like:

x = r'\\"'
# hello there

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9404