[WIP] Improvements to plugin/python_setup.vim

The script should notify on errors, as discussed in issue #1179.

The script executes the python interpreter less times now, too, to improve startup time.
My test was skewed because I wasn't piping the output, though. It is not that slow, but `python --version` is still faster.

     > ~ time python2 -c "import sys; sys.stdout.write(str(sys.version_info[0]) + '.' +     str(sys.version_info[1]))" >> speed-test 
    0.01user 0.00system 0:00.02elapsed 88%CPU (0avgtext+0avgdata 6100maxresident)k
    0inputs+8outputs (0major+763minor)pagefaults 0swaps 
    > ~ time python2 -c "import sys; sys.stdout.write(str(sys.version_info[0]) + '.' + str(sys.version_info[1]))"
    2.70.02user 0.00system 0:00.02elapsed 89%CPU (0avgtext+0avgdata     6144maxresident)k
    0inputs+0outputs (0major+766minor)pagefaults 0swaps