fdf_error -- Return error description for fdf error code
Description
string fdf_error ( [int error_code] )
fdf_error() returns a textual description for
the fdf error code given in error_code.
The function uses the internal error code set by the last operation
if no error_code is given, so
fdf_error() is a convenient shortcut for
fdf_error(fdf_errno()).