[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.go

@@ -143,6 +143,10 @@ func (me *InviteProvider) DoCommand(a *App, args *model.CommandArgs, message str
 		var text string
 		if err.Id == "api.channel.add_members.user_denied" {
 			text = args.T("api.command_invite.group_constrained_user_denied")
+		} else if err.Id == "store.sql_team.get_member.missing.app_error" {
+			text = args.T("api.command_invite.user_not_in_team.app_error", map[string]interface{}{
+				"Username": userProfile.Username,
+			})
 		} else {
 			text = args.T("api.command_invite.fail.app_error")
 		}

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,
 		},
 		{

i18n/en.json

@@ -798,6 +798,10 @@
     "id": "api.command_invite.user_already_in_channel.app_error",
     "translation": "{{.User}} is already in the channel."
   },
+  {
+    "id": "api.command_invite.user_not_in_team.app_error",
+    "translation": "@{{.Username}} is not a member of the team."
+  },
   {
     "id": "api.command_invite_people.permission.app_error",
     "translation": "You don't have permission to invite new users to this server."