new util that also passes back the returncode
test for get_output_error_code
updated tests to use get_output_error_code
allow for List traitlets to come from command line
all nbconvert tests pass with pandoc
the last set of pandoc-dependent test
move ipython command line logic to one place

it keeps getting repeated, more elegant to move it out into the call
moved get_ipython_cmd into separate function
use new get_ipython_cmd functionality
pass repr of a list on command line
moved TemporaryWorkingDir into utils.tempdir
test for TemporaryWorkingDirectory
simplified copy_files logic
minor typos
cleanup unused bits
Merge pull request #3834 from ivanov/nbconvert-better-tests

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.