Hide the message widget when the URL changes

Most error messages are only relevant when trying to open a URL, e.g.,
they inform the user that opening the URL failed. After the next
successful URL change, these errors are not relevant any more.
Therefore, it makes sense to hide the message widget automatically to
prevent that the user is forced to close it manually.

BUG: 312872
FIXED-IN: 4.11.0
REVIEW: 110369