Allow cut&copy from error messages

Sometimes is is useful to paste parts of the error message into the code, e.g. the lists of cases that are not matched in this example:

However, currently the whole error message is one "button", one cannot select any text there.

No file or test is named in the issue. Start by locating the VS Code extension code that renders error messages, then check how the message is presented as a single button. Done means users can select and copy part of an error message, including unmatched-case lists, rather than only treating the whole message as one control.

Written by the indexing model from the issue text.