Warn, don't raise, a double started Pretender

.jshintrc

@@ -6,6 +6,9 @@
     "define",
     "process"
   ],
+  "globals": {
+    "console":false
+  },
   "expr": true,
   "proto": true,
   "strict": true,

pretender.js

@@ -261,8 +261,9 @@ function interceptor(pretender, nativeRequest) {
   FakeRequest.prototype = proto;
 
   if (nativeRequest.prototype._passthroughCheck) {
-    throw new Error('You created a second Pretender instance while there was already one running. ' +
-          'Running two Pretender servers at once will lead to unexpected results!' +
+    console.warn('You created a second Pretender instance while there was already one running. ' +
+          'Running two Pretender servers at once will lead to unexpected results and will ' +
+          'be removed entirely in a future major version.' +
           'Please call .shutdown() on your instances when you no longer need them to respond.');
   }
   return FakeRequest;

test/creation_test.js

@@ -2,26 +2,6 @@ var pretender;
 var describe = QUnit.module;
 var it = QUnit.test;
 
-describe('pretender creation - without shutdown', function(config) {
-  var secondPretender;
-
-  config.beforeEach(function() {
-    pretender = new Pretender();
-  });
-
-  config.afterEach(function() {
-    pretender.shutdown();
-  });
-
-  test('an error is thrown when you start a new pretender while another one is running', function(assert) {
-    var message = 'You created a second Pretender instance while there ' +
-                  'already one running. Running two Pretender servers at once will lead to unexpected results!';
-    assert.throws(function() {
-      new Pretender();
-    }, message);
-  });
-});
-
 describe('pretender creation', function(config) {
   config.afterEach(function() {
     if (pretender) {