mw.ViewPageTarget: Surface error messages from core edit api

Previously we only looked at error info/code from the root of
the API response, not the ones from the root of the response
that action=visualeditoredit forwards from action=edit.

This changes the message for e.g. AbuseFilter from:

 > Error: Invalid error code

to:

 > Error: Hit AbuseFilter: [name of triggered filter]

Also changed default error for other reasons (e.g. hooks of other
extensions that we don't yet have specific support for) to "Unknown error"
instead of "Invalid error code".

Bug: 50472
Change-Id: I3b32eddafd8fff83f323606f9922ac60b5d3b58e