Use hasErrorsOrWarningsToDisplay for Syntastic

Reverts PR #759 based on feedback from @lcd047 in scrooloose/syntastic#950
Merge pull request #764 from fitztrev/syntastic-update

Use hasErrorsOrWarningsToDisplay for Syntastic
Use nothing on right side of `_powerline_prompt`

It should be more robust in case every PROMPT_COMMAND setter cares about
previous commands

Closes #765
Merge branch 'no-PROMPT_COMMAND-colon' into develop