Return an error code from getErrorMessage()

A lot of callback functions of SIGNDOC_SignRSA etc. return a a single value (such as false) for all types of errors. To pass a SignDoc SDK error code back to SignDoc SDK, append "[[rc]]" to the error message returned by getErrorMessage() where rc is the SignDoc SDK error code. SIGNDOC_Document_addSignature() will return that error code.