Add boundary checks to FileBuf

[mikowals]

I added a getter to catch errors when reading and made the error messages from moving more accurate.

While doing this I thought about grouping more of the code inside the FileBuf struct. read_file could be made an __init__ and the various read_val... methods could be moved into the struct also. this would mostly make the code quicker to find. I decided if that was desirable it was best in a separate request.

[tairov]

thanks. Moving methods inside FileBuf struct sounds great, happy to checkout another pr 👍