[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.
    echoerr expand('<sfile>').": python: failed to import neovim module."

Avoid contractions (`can't`) in system messages, and I think "python interpreter" is a bit ceremonious except in the case of line 28. "python" will do.