hdf_open - error reporting

```
IDL> a=hdf_open(/create)

a=hdf_open(/create)
                   ^
% HDF_OPEN: Incorrect number of arguments.
```

```
GDL> a=hdf_open(/create)
% HDF_OPEN: Variable is undefined: <Undefined>
% Execution halted at: $MAIN$  
```