split up missing and invalid certificate errors (closes #32)

Author: smhg

src/MessageValidator.php

@@ -86,6 +86,11 @@ public function validate(Message $message)
         // Get the certificate.
         $this->validateUrl($message['SigningCertURL']);
         $certificate = call_user_func($this->certClient, $message['SigningCertURL']);
+        if ($certificate === false) {
+            throw new InvalidSnsMessageException(
+                "Cannot get the certificate from \"{$message['SigningCertURL']}\"."
+            );
+        }
 
         // Extract the public key.
         $key = openssl_get_publickey($certificate);

tests/MessageValidatorTest.php

@@ -88,6 +88,17 @@ function () {
         $this->assertTrue($validator->isValid($message));
     }
 
+    /**
+     * @expectedException \Aws\Sns\Exception\InvalidSnsMessageException
+     * @expectedExceptionMessageRegExp /Cannot get the certificate from ".+"./
+     */
+    public function testValidateFailsWhenCannotGetCertificate()
+    {
+        $validator = new MessageValidator($this->getMockHttpClient(false));
+        $message = $this->getTestMessage();
+        $validator->validate($message);
+    }
+
     /**
      * @expectedException \Aws\Sns\Exception\InvalidSnsMessageException
      * @expectedExceptionMessage Cannot get the public key from the certificate.