This PR fixes a few issues with nbconvert tests

The code for testing 'ipython nbconvert' prior to this PR did not work as
intended, and simply swallowed errors when pandoc wasn't installed, for example.

This PR adds a new `get_output_error_code` utility for easier checking of error
(looking at return code as opposed to the contents of stdout for the word
'error'). 

This new machinery is leveraged when calling nbconvert during tests.

~~Additionally, tests that utilized `--notebook=` were failing because that
Configurable was specified as a List, but everything coming from the command
line is a string (unicode). I've modified traitlets such that values which are
strings, but should be Containers (e.g., Lists), are split on commas and turned
into lists. One limitation of this is that filenames containing commas won't be
properly specified from the command line, but it vastly improves the CLI user
experience for specifying files (see the changes in the test which use
`--notebooks`) (pinging @minrk for feedback on this portion, specifically)~~ 

Got feedback from @minrk on how to pass lists from command line, punting on a simpler implementation for now

Pinging @jdfreder as I'm generally stomping around in his neck of the woods