Try to print MTA-STS missing DNS policy error reason

When proper (not self-signed) TLS certificate is not provisioned on the domain yet,
MiaB prints only:

MTA-STS policy is missing: STSFetchResult.NONE

which may confuse the administrator, as .well-known/mta-sts.txt
file is already present and opens correctly.

Print more human-friendly reason for this case.

Author: ValdikSS

management/status_checks.py

@@ -660,7 +660,16 @@ def check_mail_domain(domain, env, output):
 			else:
 				output.print_error("MTA-STS policy is present but has unexpected settings. [{}]".format(policy[1]))
 		else:
-			output.print_error("MTA-STS policy is missing: {}".format(valid))
+			certmessage = ''
+			cert = get_ssl_certificates(env).get(domain)
+			if not cert:
+				certmessage = "No TLS certificate provisioned for this domain"
+			else:
+				cert_status = check_certificate(domain, cert['certificate'], cert['private-key'])
+				if cert_status[0] != 'OK':
+					certmessage = "TLS certificate is not valid"
+
+			output.print_error("MTA-STS policy is missing: {}".format(certmessage if certmessage else valid))
 
 	else:
 		output.print_error("""This domain's DNS MX record is incorrect. It is currently set to '%s' but should be '%s'. Mail will not