Make incoming mail handling more robust / correct

Summary:
* Properly handle when no mail headers at all can be parsed
* Properly handle when mail headers can be parsed but no subject line can be found

```
EXCEPTION: (RuntimeException) Undefined index: subject
```

Closes T15769

Test Plan: See T15769

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: 20after4, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15769

Differential Revision: https://we.phorge.it/D25565

externals/mimemailparser/MimeMailParser.class.php

@@ -308,10 +308,10 @@ class MimeMailParser {
 	 * @param $part Array
 	 */
 	private function getPartHeaders($part) {
-		if (isset($part['headers'])) {
+		if (isset($part['headers']) && $part['headers']) {
 			return $part['headers'];
 		}
-		return false;
+		throw new Exception('MimeMailParser::getHeaders() could not parse any email headers.');
 	}
 
 	/**

scripts/mail/mail_handler.php

@@ -55,7 +55,9 @@ foreach (array('text', 'html') as $part) {
 }
 
 $headers = $parser->getHeaders();
-$headers['subject'] = phutil_decode_mime_header($headers['subject']);
+if (array_key_exists('subject', $headers)) {
+  $headers['subject'] = phutil_decode_mime_header($headers['subject']);
+}
 $headers['from'] = phutil_decode_mime_header($headers['from']);
 
 if ($args->getArg('process-duplicates')) {