Option to suppress error in python-less vim

If the g:powerlineNoPythonError variable is set don't issue an error
message even if vim doesn't have the required Python support.  This
allows a common set of configuration files to be used across systems
where some of the copies of vim don't meet the requirements.
Merge branch 'no_python_warning' into run

* no_python_warning:
  Option to suppress error in python-less vim