ole32: Check the small block size limit of storage files.

ole32: Check the small block size limit of storage files.

This value is stored in the storage file header. We currently hard-code it to
0x1000. I don't expect to see files in the wild with other values, but
according to MS this is a valid configuration. For now, just fail if we see
another value.

I've also upgraded the message for unexpected values in storage file headers
to a fixme, since they are valid according to MS.