boot: Remove use of EFI_ERROR

boot: Remove use of EFI_ERROR

The macro is ugly and annoying to use and provides no real benefit. The
only reason to use it would be to allow warnings to go through. But any
EFI APIs we call do not return warning status codes or we do not check
the return value anyway. The only other case would be BS->StartImage,
where we already treat anything other than EFI_SUCCESS as an error
anyway.

This also helps the compiler and code analyzers to better reason about
the code. In particular, this can help reduce use of uninitialized
variable warnings.