[PATCH 0 of 2 V2] run pyflakes on all python files

[Simon Heimberg]

The feedback to the previous version is integrated.
  * first test more files and write their exceptions, then fix it (Patch 1 was after 2)
  * redirected stderr of hg manifest to /dev/null (in Patch 1)
  * drop some debug comments instead of rewriting it (in Patch 2)

Not sure if rejecting all stdout lines is a good idea. When anything would 
really fail it would probably be invisible. But I do not see how filter stderr.