H5Aget_storage_size, H5Dget_storage_size: Fix error return

Fix this:
> Warning: **H5Dget_storage_size()** does not differentiate between 0 (zero), the value returned for the storage size of a dataset with no stored values, and 0 (zero), the value returned to indicate an error.

**H5Aget_storage_size()** has the same problem, except it is undocumented in the current reference manual.

Please add error returns for these functions. Some applications need to know the error status. This will be tricky because the current return type is an unsigned integer, and nothing else. Therefore an error sentinel such as -1 is not possible, and any positive sentinel would be misleading.

It seems to me that the most cautious strategy will be to introduce a second return item for a function status code. Yes this means a second argument, and versioned functions.

Changing the primary return type from unsigned to signed, and using a negative sentinel, might also be okay. This would also need versioned functions to do it properly.