Expose libgit2 error code to clients when a promise fails

[rcjsuen]

This PR fixes up the C++ templates so that libgit2's error code is exposed on the JavaScript side by analyzing the rejected promise.

return git.Merge.base(repo, parents[0], parents[1])
.then(function(base) {
  // do whatever
})
.catch(function(err) {
  console.log(err);
  console.log(err.msg);
  console.log(err.errno);
});

// err
{ [Error: No merge base found] errno: -3 }
// err.msg
No merge base found
// err.errno
-3

[johnhaley81]

Just conflicts that need to be resolved then this is GTG.