Modify regex to detect specific flake8 errors.

This change causes SublimeLinter to color code errors differently than warnings.  

This list of errors is highly subjective, but I believe the following pep8/pyflakes reports are errors, while everything else is a warning.

Pyflake Errors:
- F402    import module from line N shadowed by loop variable
- F404    future import(s) name after other statements
- F812    list comprehension redefines name from line N
- F823    local variable name ... referenced before assignment
- F831    duplicate argument name in function definition
- F821    undefined name name

Pep8 Errors:
- E112    expected an indented block
- E113    unexpected indentation
- E901    SyntaxError or IndentationError
- E902    IOError

I also want to note that I'm not very familiar with F822, "undefined name name in __all__".  When I've encountered it, it has usually been a false-positive.  I'm leaving it as a warning, but this is probably more of a bug than an intended design on pyflakes part.  It's because of issues like this that I feel my categorization of error vs warning is highly subjective.