Database errors are not displayed helpfully

[rhigman]

When a user attempts to submit data which violates a database constraint, the error message displayed is missing the actual database error; it only contains the error "detail".

E.g. on attempting to create a contribution with contribution_ordinal 0:

The database produces two messages, an ERROR followed by a DETAIL. While not very layperson-friendly, the ERROR message at least gives a clue as to which field was invalid (i.e. contribution_ordinal); the DETAIL message does not.

The notification in the GUI seems to think it is displaying both, the ERROR portion under "With error:" and the DETAIL portion under "and content:". However, the "With error:" section is consistently left blank, in all notifications on all forms and for all types of error.

Logging indicates this may be a third-party (Yew) bug, as the notification is displaying the contents of a FetchError of type DeserializeError { error: String, content: String }, but the error portion is already empty when the FetchError reaches the app:

(We should ideally make the error notifications much more user-friendly in general, but being able to see both halves of the database message would be a start!)