[c-ffi] Add a new error-condition result type.

This helps writing better wrappers for C libraries where the
return value is used to signal if the function did error.
[doc] Add documentation about error-result.
[doc] Update release notes with latest changes to C-ffi.
Merge pull request #803 from fracek/c-ffi-error-condition

[c-ffi] Add a new error-condition result type.