Change signature of H5Tget_member_offset()

H5Tget_member_offset() can fail, but it currently has no coherent way to return a failure, since the return value of the function is the offset, the function returns 0 on failure, and 0 is also a valid offset. We should modify the function to be able to unambiguously return failure.

We should probably do this by moving the offset returned to a separate parameter and returning herr_t:

`herr_t H5Tget_member_offset2(hid_t type_id, unsigned membno, size_t *offset)`

But if that is objectionably different from other similar functions we could instead return ssize_t and use -1 as the error value, though this could cause latent compatibility issues. Either approach will require versioning the function.