This commit changes the internal file system implementation for reading
data from stdin, it now returns an error when no bytes could be read. I
think it's worth failing in this case, the user instructed restic to
read some data from stdin, and no data was read at all. Maybe it was in
a pipe and some earlier stage failed.
See #2135 for a short discussion.
This commit changes how the worker goroutines for saving e.g. blobs
interact. Before, it was possible to get stuck sending an instruction to
archive a file or dir when no worker goroutines were available any more.
This commit introduces a `done` channel for each of the worker pools,
which is set to the channel returned by `tomb.Dying()`, so it is closed
when the first worker returned an error.
This commit changes the archiver so that low-level errors saving data to
the repo are returned to the caller (instead of being handled by the
error callback function). This correctly bubbles up errors like a full
temp file system and makes restic abort early and makes all other worker
goroutines exit.
