Error handling improvements

[heyheyjp]

Fixes https://github.com/LearnersGuild/idm/issues/100.

• [Server] Pushes unhandled errors all the way back up to be logged and otherwise managed by the catch-all express and graphql error handling middleware (for the server app; standalone scripts also handle errors at their top level as needed). Having a central place for server error handling will make them easier to manage and help ensure that errors of similar types will be handled consistently.
• [Server] With a centralized server error handling location, errors are now (minimally) inspected, logged, and the response tailored as needed to control information returned to clients in the event of an internal server error.

• [Client] Fixes error message display in app snackbar.

  Previously:

  

  Now:

  

[heyheyjp]

@jeffreywescott: would be awesome if you could give this some eyes, let me know what you think. Not super urgent, but I think it'll be important to pull these changes (in whatever their final state is) in before going live.

[jeffreywescott]

Love this. Made a few comments, none of which necessarily need to block the merge, but I thought I'd give you a chance to at least see them before merging. Definitely wondering about the MongoError and the best way to match DB errors.

[heyheyjp]

@jeffreywescott @bundacia: any outstanding concerns?