Improve replication document error when user_ctx or auth headers are not parsed correctly

[nickva]

Adding something like

"user_ctx": null

to the replication document results in the state failed which is correct but the error reason is: "{error,{case_clause,null}}" which is not the best.

Same goes for using a null value for a header in a source/target endpoint object.

[nickva]

If not an error with a specific field identifier and value being mentioned at least it would be nice to have something like "document parsing failed".

[mikerhodes]

This is almost something which would benefit from a (the horror!) schema validation to pre-flight this and catch lots of errors in one go?