Add a stringer for ErrorCode.
Add a stringer for ErrorCode.

With this change we go from hard to reason about errors like:

  Error 65537: The GLFW library is not initialized

To more clean ones:

  NotInitialized: The GLFW library is not initialized

Note: can't use go:generate + stringer because they are C constants.

go-gl/glfw,jonbuckley33/glfw3,gonutz/glfw,james4k/glfw,go-gl/glfw,james4k/glfw,jonbuckley33/glfw3,go-gl/glfw,gonutz/glfw