[PATCH] error_code for dynamically generated error messages

[Michael Spencer]

One of the intents of ErrorOr was for this type of stuff to live there. This functionality was removed from ErrorOr because it was unused, but I would rather add it back to ErrorOr in some way than extend error_code like this.

I would also actually like to change ErrorOr to behave more like: https://github.com/boostcon/cppnow_presentations_2014/blob/master/files/expected.pdf?raw=true

The main part is to make the error type changeable and figure out a good default. Another feature that would make using ErrorOr easier is the monadic syntax with map.

http://reviews.llvm.org/D3881