Added comment

app/src/main/java/eu/faircode/email/Core.java

@@ -499,6 +499,7 @@ class Core {
                             // Fetch: NO [SERVERBUG] SELECT Server error - Please try again later
                             // Fetch: NO [SERVERBUG] UID FETCH Server error - Please try again later
                             // Fetch: NO Invalid message number (took n ms)
+                            // Fetch: BAD Internal Server Error
                             // Move: NO Over quota
                             // Move: NO No matching messages
                             // Move: NO [EXPUNGEISSUED] Some of the requested messages no longer exist