Added comment

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

@@ -519,6 +519,7 @@ class Core {
                             // Fetch UID: NO Some messages could not be FETCHed (Failure)
                             // Fetch UID: NO [LIMIT] UID FETCH Rate limit hit.
                             // Fetch UID: NO Server Unavailable. 15
+                            // Fetch UID: NO [UNAVAILABLE] Failed to open mailbox
                             // Move: NO Over quota
                             // Move: NO No matching messages
                             // Move: NO [EXPUNGEISSUED] Some of the requested messages no longer exist