init: pass errors from one Result<T> to another better

Result<T> currently has two problems,
1) A failing Result<T> cannot be easily constructed from a Result<U>'s
error.
2) errno is lost when passing .error() through multiple Result<T>'s

This change fixes both problems having Result<T>::error() return a
ResultError class that contains the std::string error message and int
errno.

It additionally has ostream operators to continue to allow printing
the error string directly to an ostream and also to pass the errno
through to another Result<T> class via Error() creation.

Lastly, it provides a new constructor for Result<T> for ResultError,
such that a Result<T> can be constructed from Result<U>::error().

Test: boot bullhead, init unit tests
Change-Id: Id9614b727cdabd2f5498b0da0e598e9aff7d9ae0