Expose last decryption error of an event that failed to be decrypted

src/models/event.js

@@ -140,6 +140,10 @@ module.exports.MatrixEvent = function MatrixEvent(
      * attempt may succeed)
      */
     this._retryDecryption = false;
+
+    // The last DecryptionError thrown when failing to decrypt the event, or
+    // null if the event never failed to decrypt.
+    this._lastDecryptionError = null;
 };
 utils.inherits(module.exports.MatrixEvent, EventEmitter);
 
@@ -332,6 +336,16 @@ utils.extend(module.exports.MatrixEvent.prototype, {
             this._clearEvent.content.msgtype === "m.bad.encrypted";
     },
 
+    /**
+     * Get the last error thrown if the event failed to be decrypted.
+     *
+     * @return {module:crypto/algorithms/base.DecryptionError?} The last error
+     * thrown if the event failed to be decrypted or `null` if it never failed.
+     */
+    getLastDecryptionFailureError: function() {
+        return this._lastDecryptionError;
+    },
+
     /**
      * Start the process of trying to decrypt this event.
      *
@@ -422,6 +436,8 @@ utils.extend(module.exports.MatrixEvent.prototype, {
                     return;
                 }
 
+                this._lastDecryptionError = e;
+
                 // see if we have a retry queued.
                 //
                 // NB: make sure to keep this check in the same tick of the