python/flake8: assume all outputs are errors - not warnings

The output of flake8 is ambiguous as to whether results should be
interpreted as warnings or errors. I have changed it to assume errors to
match the python/pyflakes checker.

In future we may want to change this or add items to the errorformat for
some specific warnings if they are annoying.

This fixes #203.

syntax_checkers/python/flake8.vim

@@ -26,6 +26,6 @@ endfunction
 
 function! SyntaxCheckers_python_GetLocList()
     let makeprg = 'flake8 '.g:syntastic_python_checker_args.' '.shellescape(expand('%'))
-    let errorformat = '%E%f:%l: could not compile,%-Z%p^,%W%f:%l:%c: %m,%W%f:%l: %m,%-G%.%#'
+    let errorformat = '%E%f:%l: could not compile,%-Z%p^,%E%f:%l:%c: %m,%E%f:%l: %m,%-G%.%#'
     return SyntasticMake({ 'makeprg': makeprg, 'errorformat': errorformat })
 endfunction