Clean up Mattermost team creation error handling
If trying to create a team name that already exists, you get:
The form contains the following error:
- Mattermost team A team with that domain already exists
![image](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
It's at least missing a colon or something to separate the field name from the error message.