trail create: refuse a repository with Trails off before pushing

On a repository whose Trails setting is off, entire trail create pushed the branch, then relayed the server's 422 under "inspect trails before retrying; creation may have succeeded", although nothing was created and nothing said to turn Trails on.

Read the repository's trails_enabled first and refuse with the setting's name before publishing the branch; a failed read still lets the server answer. A create the server rejects outright (4xx other than timeout or rate limit) now says no trail was created.