On FileHandle Read now just returns the data hash and error
This makes it more obvious that you should handle the error, rather than hiding
it all in a struct.
We also change the way we get and return content, the main source.File
constructs now hold a FileHandle that then updates on invalidation
Change-Id: I20be1b995355e948244342130eafec056df10081
Reviewed-on: https://go-review.googlesource.com/c/tools/+/180417
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
4d9ae51c24