[MM-41785] Show error in invitation popin when email can't be send (#19617)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/email/errors.go

@@ -10,4 +10,5 @@ var (
 	NoRateLimiterError     = errors.New("the rate limit could not be found")
 	SetupRateLimiterError  = errors.New("the rate limiter could not be set")
 	RateLimitExceededError = errors.New("the rate limit is exceeded")
+	SendMailError          = errors.New("could not send the email")
 )