HDF5File.attributes("foo") does not raise an error if "foo" is not a dataset

[Johan Hake]

from dolfin import *
f = HDF5File(mpi_comm_world(), "jada.h5", "w")
attr = f.attributes("foo") # Should raise error

[Johan Hake]

fixed by: 66bc8000d2b5333930c4ba245e30a49a113d8967