fix: add 422 Error class and status_code to DataCiteError

[alee]

use a defaultdict to map status codes to error classes, defaults to DataCiteServerError for status_code >= 500 and DataCiteRequestError for all others

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[tmorrell]

Hi @alee. Sorry for the delay in looking at this. It looks like status_code needs to be added to all the tests in order for this improvement to go through. It also might not be a bad idea to run the tests against the live DataCite endpoint and see what the actual response is. I think a bunch of the test responses don't match what DataCite currently uses. I'll also add this to my list of things to work on, but not sure when I'll get to it.