Add default_log_stream Powerline attribute, make it sys.stdout in Vim

Reason for changes in VimPowerline:

    try
        python import sys; sys.stderr.write('abc')
    catch
        echomsg v:exception
    endtry

reports an exception. This should not happen because stack trace that ought to 
be printed by the logger will not be shown to the user, making it hard to 
determine the error and turning some recoverable errors to non-recoverable when 
using powerline.vim.
Fix some errors in troubleshooting code
Make it immune to the absense of powerline_appended_path
In vim tests check for printed messages
Merge pull request #1056 from ZyX-I/vim-troubleshooting

Improve vim troubleshooting