Provide additional hints when trac-admin repository commands are not provided a valid repository name

[Ryan J Ollos]

From reports on the MailingList and personal experience, it seems users mistakenly input "repository <path>" rather than "repository <name>" for the trac-admin repository commands:

$trac-admin tracenvs/proj-1.3 help repository
repository add <repos> <dir> [type]

    Add a source repository

repository alias <name> <target>

    Create an alias for a repository

repository list 

    List source repositories

repository remove <repos>

    Remove a source repository

repository resync <repos> [rev]

    Re-synchronize trac with repositories

repository set <repos> <key> <value>

    Set an attribute of a repository

repository sync <repos> [rev]

    Resume synchronization of repositories

We could provide additional hints, like the following:

$trac-admin tracenvs/proj-1.3 repository sync "notfound" 10
TracError: Repository "notfound" doesn't exist.

Use 'repository list' to view the valid repository names.

trac/versioncontrol/admin.py

@@ -128 +128 @@
                 reponame = ''
             repos = rm.get_repository(reponame)
             if repos is None:
-                raise TracError(_("Repository \"%(repo)s\" doesn't exist",
+                raise TracError(_("Repository \"%(repo)s\" doesn't exist.\n"
+                                  "\nUse 'repository list' to view the "
+                                  "valid repository names.",
                                   repo=reponame or '(default)'))
             if rev is not None:
                 repos.sync_changeset(rev)

Similar changes would need to be made for the other trac-admin repository commands. Alternatively we could list the repository names when the error occurs.