Return an error code from getErrorMessage()

A lot of callback functions of de::softpro::doc::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. de::softpro::doc::SignDocDocument::addSignature() will return that error code.