clean up error formatting when the operation times out

Change the logic for errors so it no longer replaces the timeout
error stack with the original error's stack. This way the timeout
is actually a proper error, while the original failure is preserved
as the `failure` member of the timeout error.

lib/bluebird-retry.js

@@ -92,10 +92,9 @@ function retry(func, options) {
                         err.failure = failure;
                     }
 
-                    var timeout = new Error('operation timed out after ' + (now - start) + ' ms, ' + tries + ' tries' + ' failure: ' + err.message);
+                    var timeout = new Error('operation timed out after ' + (now - start) + ' ms, ' + tries + ' tries with error: ' + err.message);
                     timeout.failure = err;
                     timeout.code = 'ETIMEDOUT';
-                    timeout.stack = err.stack;
                     return Promise.reject(timeout);
                 } else {
                     var delay = interval - (now - tryStart);

test/bluebird-retry.spec.js

@@ -104,7 +104,9 @@ describe('bluebird-retry', function() {
                         throw new Error('unexpected success');
                     })
                     .caught(function(err) {
-                        expect(err.message).match(/operation timed out/);
+                        expect(err).match(/operation timed out.*not yet/);
+                        expect(err.message).match(/operation timed out.*not yet/);
+                        expect(err.failure).match(/not yet/);
                         expect(err.failure.message).match(/not yet/);
                         expect(countSuccess.count).equal(10);
                     })
@@ -120,7 +122,6 @@ describe('bluebird-retry', function() {
                         throw new Error('unexpected success');
                     })
                     .caught(function(err) {
-                        console.log(err.stack)
                         expect(err).match(/operation timed out.*something bad happened/);
                         expect(err.message).match(/something bad happened/);
                         expect(err.message).match(/operation timed out/);