lei/store: (more) synchronous non-fatal error output

lei/store: (more) synchronous non-fatal error output

Since every command that writes to lei/store calls ->done
to commit its output, we can rely on that to return a
pathname for a readable file with errors in it.

Errors can still get crossed up if multiple lei commands
are writing to the store at once, but reduces the delay
in seeing them and ensures it won't get seen when somebody
is attempting to use shell completion.