render custom HTML for error pages
catch pandoc failures in nbconvert handlers
add error page templates
add error css
allow notebook to start without nbconvert

catches import / key errors and turns them into proper http errors
never import pigments at module level in nbconvert

ensures things are always importable, and delays ImportErrors until they are actually relevant.
delay jinja2 imports to runtime in nbconvert

same treatment as pygments in previous commit
turn missing dependencies in nbconvert to 500 errors

pygments is the only such example at this time
be more specific about unknown status codes

per review
remove unnecessary conversion of newline to br tag

relic from when I was using div, not pre
Merge pull request #4735 from minrk/better-errors

add some HTML error pages