Raise a named exception when pandoc is missing

In the normal case this will case the interpriter to exit just like
sys.exit(...), but it will make it possible for callers to explicitly
catch the exception too.

Also, removed parens from the pandoc URL so it's easier to
triple-click-copy-and-paste.
Catch ConversionExceptions in main nbconvert loop
Use 'self.exit' instead of 'sys.exit'
Exit with status 1, not -1
Fine, keep your newlines
Remove exception comments
Merge pull request #3746 from wolever/pandoc-missing-exception

Raise a named exception when pandoc is missing