change: Olm exception handling

lib/encryption/encryption.dart

@@ -392,7 +392,8 @@ class DecryptException implements Exception {
   DecryptException(this.cause, [this.libolmMessage]);
 
   @override
-  String toString() => cause;
+  String toString() =>
+      cause + (libolmMessage != null ? ': $libolmMessage' : '');
 
   static const String notEnabled = 'Encryption is not enabled in your client.';
   static const String unknownAlgorithm = 'Unknown encryption algorithm.';

lib/encryption/olm_manager.dart

@@ -305,6 +305,7 @@ class OlmManager {
           try {
             plaintext = session.session.decrypt(type, body);
           } catch (e) {
+            // The message was encrypted during this session, but is unable to decrypt
             throw DecryptException(
                 DecryptException.decryptionFailed, e.toString());
           }