more graceful handling of retrieve of unknown ID

[alaniwi]

A retrieve of an unknown erratum id, for example:

https://errata.es-doc.org/1/issue/retrieve?uid=01234567-89ab-cdef-0123-456789abcdef

fails with an HTTP 500 status, with content:

{"errorCode": 999, "errorMessage": "'NoneType' object has no attribute 'to_dict'", "errorType": "AttributeError", "errorField": "--"}

It would probably be better to fail with a 400 ("bad request") in one form or another.

[AtefBN]

I agree Alan, I'll note this to be part of the errata coding sprint @momipsl and I will be holding end of this month.

[alaniwi]

@AtefBN Thanks.