Improve Exception message on 403 response

lib/SparkPost/APIResource.php

@@ -193,6 +193,9 @@ private function callResource( $action, $resourcePath=null, $options=[] ) {
       if ($statusCode < 400) {
         return json_decode($response->getBody()->getContents(), true);
       }
+      elseif ($statusCode === 403) {
+        throw new APIResponseException('Request forbidden.  Does this API Key have the correct SparkPost permissions?');
+      }
       elseif ($statusCode === 404) {
         throw new APIResponseException('The specified resource does not exist', 404);
       }

test/unit/APIResourceTest.php

@@ -111,6 +111,21 @@ public function testAdapter404Exception() {
     }
   }
 
+  public function testAdapter403Exception() {
+    try {
+      $responseMock = Mockery::mock();
+      $this->sparkPostMock->httpAdapter->shouldReceive('send')->
+      once()->
+      andReturn($responseMock);
+      $responseMock->shouldReceive('getStatusCode')->andReturn(403);
+
+      $this->resource->get('test');
+    }
+    catch(\Exception $e) {
+      $this->assertRegExp('/Request forbidden/', $e->getMessage());
+    }
+  }
+
   public function testAdapter4XXException() {
     try {
       $testBody = ['errors'=>['my'=>'test']];