mw.Target: Use saveError's third parameter for response data

This parameter was documented as "HTTP status text" but this
was incorrectly copied from other events. The actual code
never passed anything other than null.

Now documenting it as API response data (if available, else null)

Change-Id: I88c8c525f3ca53fd47de634aec469be372884cfc
Merge "mw.Target: Use saveError's third parameter for response data"