fix: ensure breaker.fire() returns rejected promise when fallback fails

Prior to this change, the user was required to handle promise rejections
from a fallback function during the fallback event. Now, the user may
choose to handle the rejected promise in the event handler, or in the
promise returned from `breaker.fire()`.

Fixes: #100

lib/circuit.js

@@ -410,8 +410,9 @@ function handleError (error, circuit, timeout, args, latency, resolve, reject) {
 
 function fallback (circuit, err, args) {
   if (circuit[FALLBACK_FUNCTION]) {
+    const result = circuit[FALLBACK_FUNCTION].apply(circuit[FALLBACK_FUNCTION], args);
+    if (result instanceof Promise) return result;
     return new 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

test/test.js

@@ -581,14 +581,9 @@ test('CircuitBreaker fallback as a rejected promise', (t) => {
   const breaker = cb(passFail, options);
   breaker.fallback(() => Promise.reject(new Error('nope')));
 
-  breaker.on('fallback', (resultPromise) => {
-    resultPromise
-      .then(t.fail)
-      .catch((e) => t.equals('nope', e.message))
-      .then(t.end);
-  });
-
-  breaker.fire(input).catch(() => {});
+  breaker.fire(input).then(t.fail).catch(e => {
+    t.equals('nope', e.message);
+  }).then(t.end);
 });
 
 test('CircuitBreaker fallback as a CircuitBreaker', (t) => {