better handling of missing workflow inputs

[zackgalbreath]

Currently if your workflow is missing an input connection it just says "undefined" when you attempt to run it. We should probably check that the entire workflow pipeline is properly connected when the user attempts to save it.

[zackgalbreath]

A related issue exists for analysis outputs. Suppose you have a typo in your script and your output variable is named "tabel" instead of "table". The analysis will run to completion and then tangelo-hub will respond with "KeyError: table". We need to improve our error handling in cases such as these to more accurately point our users towards what they did wrong.

[jeffbaumes]

Great idea.