warrior.py: Fix Python 3 detecting missing task

[jayvdb]

Replaces 'foo' in e with 'foo' in str(e).

Fixes #127

[coddingtonbear]

Actually, I think #130 solves this same problem, just in a slightly more pythonic way. Thanks for your patch, though!

[jayvdb]

fwiw, it was intentional to not do it that way. The other approach broke Python 2.
#130 (comment)
Can be fixed using something like https://stackoverflow.com/questions/21367320/searching-for-equivalent-of-filenotfounderror-in-python-2 .