[BUG] Handle Error Produced by Handler

[hadihammurabi]

Describe the bug
Handler has error as return value.
It can be nil if no errors.
Otherwise, error object if an error happen.

To Reproduce
Steps to reproduce the behavior:

1. Create route and bind with handler

mux.Get("/", func(rw http.ResponseWriter, r *http.Request) error {
  return errors.New("something bad")
})

2. The error is not handled.

Expected behavior
Should send HTTP response with error message and status.

[hadihammurabi]

solved at #11