In T417041 we started presenting error messages generated by the Wikibase backend directly to the user. These strings are in some cases grammatically incorrect or stylistically strange for native speakers (e.g. 'An URL scheme "fish" is not supported', 'This URL misses a scheme like "https://": test.com'). We also do not have a clear overview of which strings might possibly be presented to users and under what circumstances.

Review all user-facing Wikibase-generated error messages and check that they are presented in appropriate contexts in Wikibase, updating any messages that are clearly incorrect.