This method returns a text string describing the error represented by errno. If no text is available for the this error code, the method returns a NULL.
The string returned is owned by the object. The caller should not free it; it
will be freed when the object is destroyed, closed, or the next time the error
is reset. The error is reset whenever you call a method.
Oracle database errors (as distinct from error that occur in the use of the
class library) are reported through the OSession and ODatabase methods ServerErrorNumber and GetServerErrorText.
A valid, null terminated const char pointer on success; NULL on failure.