Better changeset error handling #195
                
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Related to #170 this PR attempts to work through better changeset error handling.
The first step was eliminating the
:uid_providercombination, and just check unique constraint on:uid. The second will be to make the changeset error handling more aware of other types of errors when checking for:bound_to_different_useror:invalid_user_id_field. I'm not sure that returning the tuple is the best way to deal with this.