Suppress CORS errors in the 'failed to join room' dialog

[turt2live]

This isn't pretty, but it does address element-hq/element-web#7526

Tested with Chrome and Firefox to ensure the message is stable enough to parse.

[dbkr]

Isn't the problem here that we are taking err.message and plonking it into the UI?

If we do need to do this kind of hack, please make sure it's flagged with a big XXX comment.

[turt2live]

The annoying part is that err.message does have valid strings in it most of the time. Some of which are actually generated within riot and not direct from the server.

Will flag this as a disgusting hack though.

[dbkr]

OK yeah - sounds like this is a quick fix for a symptom of a deeper problem. Thanks for XXXing & opening element-hq/element-web#7785 - it's a smell if we have to write hacks like this to work around bugs in our own library.