manager: simplify error handling in manager_deserialize()

manager: simplify error handling in manager_deserialize()

If a memory error occurred, we would still go through the path which sets the
error on ferror(). It is unlikely that ferror() returns true, but it's seems
cleaner to just propagate the error we already have.

The handling of fgets() returning NULL is also simplified: according to the man
page, it returns NULL only on EOF or error. So if feof() returns true, I don't
think we should call ferror() again.

While at it, let's set errno to 0 and check that it is set before returning it
as an error. The man pages for fgets() and feof() do not say anything about
setting errno.