fix: include the error when emitting the 'fallback event'

lib/circuit.js

@@ -178,7 +178,8 @@ class CircuitBreaker extends EventEmitter {
        * @event CircuitBreaker#reject
        */
       this.emit('reject', new Error('Breaker is open'));
-      return fallback(this, args) || fail(this, 'Breaker is open', args);
+      return fallback(this, 'Breaker is open', args) ||
+             fail(this, 'Breaker is open', args);
     }
     this[PENDING_CLOSE] = this.halfOpen;
 
@@ -195,7 +196,7 @@ class CircuitBreaker extends EventEmitter {
              * @event CircuitBreaker#timeout
              */
             this.emit('timeout', error);
-            resolve(fallback(this, args) || fail(this, error, args));
+            resolve(fallback(this, error, args) || fail(this, error, args));
           }, this.options.timeout);
 
         const result = this.action.apply(this.action, args);
@@ -217,28 +218,28 @@ class CircuitBreaker extends EventEmitter {
           })
           .catch((error) => {
             clearTimeout(timeout);
-            const fb = fallback(this, args);
+            const fb = fallback(this, error, args);
             if (fb) return resolve(fb);
             fail(this, error, args);
             reject(error);
           });
       });
     } catch (error) {
       clearTimeout(timeout);
-      return fallback(this, args) || fail(this, error, args);
+      return fallback(this, error, args) || fail(this, error, args);
     }
   }
 }
 
-function fallback (circuit, args) {
+function fallback (circuit, err, args) {
   if (circuit[FALLBACK_FUNCTION]) {
     return new circuit.Promise((resolve, reject) => {
       const result = circuit[FALLBACK_FUNCTION].apply(circuit[FALLBACK_FUNCTION], args);
       /**
        * Emitted when the circuit breaker executes a fallback function
        * @event CircuitBreaker#fallback
        */
-      circuit.emit('fallback', result);
+      circuit.emit('fallback', result, err);
       resolve(result);
     });
   }