message: Improve error message on channel name when pattern is invalid
diff: when="availability"
  :message="$gettext('A channel with that name already exists')"
  />
  +
  + <app-form-control-error
  + when="pattern"
  + :message="$gettext('Channel names must not contain special characters or spaces')"
  + />
  </app-form-control-errors>
  </app-form-group>
  