Improve decryption error messages

Attempt to make the decryption errors less obscure

lib/crypto/algorithms/megolm.js

@@ -458,7 +458,9 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
     if (res === null) {
         // We've got a message for a session we don't have.
         this._addEventToPendingList(event);
-        throw new base.DecryptionError("Unknown inbound session id");
+        throw new base.DecryptionError(
+            "The sender's device has not sent us the keys for this message."
+        );
     }
 
     var payload = JSON.parse(res.result);