It appears that the only macros defined in the API headers for fgdbError values are S_OK, S_FALSE, S_FALSE. The documentation and header comments list "magic numbers" for return values, eg (from CreateGeodatabase) "If the file geodatabase already exists a -2147220653 (The table already exists) error will be returned."
It would be helpful if the API header listed macros for all of the error values so that the application programmer doesn't have to do it, if we want to handle specific scenarios eg :
#define FGDB_PATH_NOT_FOUND static_cast<fgdbError>(-2147024893)
#define FGDB_TABLE_ALREADY_EXISTS static_cast<fgdbError>(-2147220653)
