handle docker exceptions when creating grader folders

[trevorcampbell]

Ran into an issue where python docker API couldn't create containers to run

nbgrader db assignment list

or

nbgrader generate_assignment --force assignment1

need to make sure rudaux stops when it encounters an error here and reports to instructor

[trevorcampbell]

We found the issue -- the config file was missing this value:

c.docker_memory = '1g' #the amount of memory for each grading thread

So this creates two TODOs for us:

• validate the rudaux_config.py file before doing anything (will make another separate issue for this)
• handle errors properly when trying to start a container

[trevorcampbell]

This partially being handled by #84 (also listed in #95). But I'll make a link from the big TODO thread in #95 to also specifically address handling container errors.