Add an example that demonstrates custom error callbacks
Add an example that demonstrates custom error callbacks

mneumann/glfw-rs,tomaka/glfw-rs,nstoddard/glfw-rs,bjz/glfw-rs,derekdreery/glfw-rs,servo/glfw-rs,bvssvni/glfw-rs,derekdreery/glfw-rs,PistonDevelopers/glfw-rs