Protect against null error object

In certain situations, library fails to parse the recurly error message (example below). This will lead to a null RecurlyAPIError object (which is fine) and an NPE in the RecurlyAPIException ctr - which is not.
This fix just guards against this, so that we have a reasonable RecurlyAPIException, and not an NPE.

Example:
RecurlyClient@832 inits recurlyError with null. An exception in line 834 would lead to an NPE in line 838.
'recurlyError.toString()' will still throw an NPE. You need 'super(recurlyError ==null? "Unspecified API Error" : recurlyError.toString());'