Fix, don't try to create '' directories
Make sure that a notebook_name is provided if writing to the filesystem
Allow for files output without an extension
Fixed call to pretty print.
pprint is the import function, not module
Fixed output to match other print statements
Fixed, should only be called if enabled
figure renamed to output
s/a/an
FIX Coalesce streams transformer, missing assignment to `last`
s/AttributeError/TypeError
s/figure/output
Merge pull request #3924 from jdfreder/backport_fixes

nbconvert: Backport fixes

cherry-picked from #3914 and #3923, which include tests that will not be merged into 1.0.