use '!=' instead of 'is not' with str
FileNotFoundError isn't exist in python2.7, use IOError instead.


Bug: chromium:948812
Change-Id: I241ff8a9321d612f2371a2a2e2deae948fa180e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1549168
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60661}