Make inaccessable private key exception more informative

Perviously setting $config['rsa_private_key'] = 'file://full/path/to/key.pem' (note two slashes instead of three) or other variations that are incompatitible with openssl_pkey_private(), would just give the user "Cannot access private key for signing", for a file that is very much accessable. The additional info I've added allows the user to go straight to the manual instead of spending time thinking it's a permissions issue.

src/XeroPHP/Remote/OAuth/SignatureMethod/RSASHA1.php

@@ -9,8 +9,9 @@ class RSASHA1 implements SignatureMethodInterface {
 
     public static function generateSignature(array $config, $sbs, $secret) {
         // Fetch the private key
-        if(false === $private_key_id = openssl_pkey_get_private($config['rsa_private_key']))
-            throw new Exception('Cannot access private key for signing');
+        if(false === $private_key_id = openssl_pkey_get_private($config['rsa_private_key'])){
+            throw new Exception("Cannot access private key for signing: openssl_pkey_get_private('${config['rsa_private_key']}') returned false");
+        }
 
         // Sign using the key
         if(false === openssl_sign($sbs, $signature, $private_key_id)) {