[MM-18830] Unhelpful error message when adding bot to a channel before adding to team (#12844)

* Fix error message

* Rename User to Username

* Add '@' to the username

app/command_invite_test.go

@@ -94,7 +94,7 @@ func TestInviteProvider(t *testing.T) {
 		},
 		{
 			desc:     "try to add a user which is not part of the team",
-			expected: "api.command_invite.fail.app_error",
+			expected: "api.command_invite.user_not_in_team.app_error",
 			msg:      basicUser4.Username,
 		},
 		{