Solve failing unittest CloudFront\SignerTest::testBadPrivateKeyPath()

There were remaining error-messages in the openssl-error-stack, resulting in an
unexpected result in the unittest.

This was solved by outputting all error-messages in Aws\CloudFront\Signer::__construct().

Another option would be to clear all error-messages in
Aws\Test\CloudFront\SignerTest::_tearDown(), so tests would no longer
interfer with eachother.

The first solution was chosen since it would output all
openssl-error-messages to the user, not only the first one,
making it more complete.

src/CloudFront/Signer.php

@@ -34,11 +34,15 @@ public function __construct($keyPairId, $privateKey, $passphrase = "")
         if (!$this->pkHandle = openssl_pkey_get_private($privateKey, $passphrase)) {
             if (!file_exists($privateKey)) {
                 throw new \InvalidArgumentException("PK file not found: $privateKey");
-            } else {
-                $this->pkHandle = openssl_pkey_get_private("file://$privateKey", $passphrase);
-                if (!$this->pkHandle) {
-                    throw new \InvalidArgumentException(openssl_error_string());
+            }
+
+            $this->pkHandle = openssl_pkey_get_private("file://$privateKey", $passphrase);
+            if (!$this->pkHandle) {
+                $errorMessages = [];
+                while(($newMessage = openssl_error_string()) !== false){
+                    $errorMessages[] = $newMessage;
                 }
+                throw new \InvalidArgumentException(implode("\n",$errorMessages));
             }
         }
     }