Improve logging when backend not available

[se0wtf]

If the backend is not available (tested wit etcd), there is no log to explain the problem, only a panic.

2018/08/09 00:42:51 error during request: context deadline exceeded
goroutine 64201 [running]:
github.com/mcluseau/autorizo/api.writeError(0xbca980, 0xf89b60, 0xc4201c8ae0)
	/go/src/github.com/mcluseau/autorizo/api/errors.go:20 +0x109
github.com/mcluseau/autorizo/api.(*API).simpleAuthenticate.func1(0xc4201c8ae0)
	/go/src/github.com/mcluseau/autorizo/api/simple.go:40 +0x71
panic(0xaa1a80, 0xf89b60)
	/usr/local/go/src/runtime/panic.go:502 +0x229
github.com/mcluseau/autorizo/api.(*API).writeAuthResponse(0xc42010bcc0, 0xc4208626c0, 0xc4201c8ae0, 0xc4202be060, 0x9, 0xc4202be0c0, 0xd)
	/go/src/github.com/mcluseau/autorizo/api/simple.go:84 +0x492
github.com/mcluseau/autorizo/api.(*API).simpleAuthenticate(0xc42010bcc0, 0xc4208626c0, 0xc4201c8ae0)
	/go/src/github.com/mcluseau/autorizo/api/simple.go:60 +0x12b
github.com/mcluseau/autorizo/api.(*API).(github.com/mcluseau/autorizo/api.simpleAuthenticate)-fm(0xc4208626c0, 0xc4201c8ae0)
	/go/src/github.com/mcluseau/autorizo/api/simple.go:13 +0x3e
github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful.(*Container).dispatch(0xc4201a4000, 0xbd31e0, 0xc420942000, 0xc42025a900)
	/go/src/github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful/container.go:249 +0x9ee
github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful.(*Container).(github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful.dispatch)-fm(0xbd31e0, 0xc420942000, 0xc42025a900)
	/go/src/github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful/container.go:95 +0x48
net/http.HandlerFunc.ServeHTTP(0xc42025c350, 0xbd31e0, 0xc420942000, 0xc42025a900)
	/usr/local/go/src/net/http/server.go:1947 +0x44
net/http.(*ServeMux).ServeHTTP(0xf6b500, 0xbd31e0, 0xc420942000, 0xc42025a900)
	/usr/local/go/src/net/http/server.go:2337 +0x130
github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful.Container.ServeHTTP(0x0, 0x0, 0x0, 0xc42025d8b0, 0x2, 0x2, 0xf6b500, 0x1, 0xf89e10, 0x0, ...)
	/go/src/github.com/mcluseau/autorizo/vendor/github.com/emicklei/go-restful/container.go:264 +0x52
net/http.serverHandler.ServeHTTP(0xc420216c30, 0xbd31e0, 0xc420942000, 0xc42025a900)
	/usr/local/go/src/net/http/server.go:2694 +0xbc
net/http.(*conn).serve(0xc42012a140, 0xbd39e0, 0xc420312cc0)
	/usr/local/go/src/net/http/server.go:1830 +0x651
created by net/http.(*Server).Serve
	/usr/local/go/src/net/http/server.go:2795 +0x27b