Fix OkOrFail<status_t> conversion ambiguities

OkOrFail<status_t> has specialized conversions for Result<int, StatusT>
to avoid ambiguous implicit conversion sequences. Since user conversion
operators sequences can be followed by integral promotion, specializing
for integral types is necessary.

Specialize ResultError<StatusT> so calling code() returns a status_t
instead of a StatusT and message() is implemented even when not carrying
a string.
Eventually, these classes should be combined.

Add equality operators for ResultError<StatusT>.

Bug: 219580167
Test: atest Errors_test.cpp
Merged-In: I14acecfd2aef33c40e79ddb091e2f4af9291d837
Change-Id: Ifb5ed3c2d3452b10901e4aeb19368d873225d9ce