[Low-quality PR] Fix user flow for "create new workspace" with bad characters

The code path I moved in this commit will not be reached AFAICT -- the server returns a 400 and reaches the exception path instead.  I've moved it and probably created other problems.  However, I can promise that currently, creating a repo with a `/` in it results in the error message "400 BAD REQUEST" and not "don't use that character".