users: Update the SSH set key error message

In a following pull request, we are expected to solve the cache timing
issue.  So, show a generic error message instead of one that suggests a
timing problem.

plinth/modules/users/forms.py

@@ -212,10 +212,7 @@ class UserUpdateForm(ValidNewUsernameCheckMixin, forms.ModelForm):
                     'ssh', ['set-keys', '--username', user.get_username(),
                             '--keys', self.cleaned_data['ssh_keys'].strip()])
             except ActionError:
-                messages.error(
-                    self.request,
-                    _('Unable to set SSH keys. Please wait a minute and then '
-                      'try again.'))
+                messages.error(self.request, _('Unable to set SSH keys.'))
 
         return user