Log when page not found (TykTechnologies#2800)

Example log:

``` 
time="Jan 13 16:07:06" level=error msg="404 page not found" request="GET /non-existing-api/get HTTP/1.1" 
```

Fixes TykTechnologies#2468

gateway/api_loader.go

@@ -683,6 +683,7 @@ func loadApps(specs []*APISpec) {
 
 	muxer := &proxyMux{}
 	router := mux.NewRouter()
+	router.NotFoundHandler = http.HandlerFunc(muxer.handle404)
 	loadAPIEndpoints(router)
 	muxer.setRouter(port, "", router)
 

gateway/proxy_muxer.go

@@ -15,8 +15,8 @@ import (
 	"github.com/TykTechnologies/again"
 	"github.com/TykTechnologies/tyk/config"
 	"github.com/TykTechnologies/tyk/tcp"
-	proxyproto "github.com/pires/go-proxyproto"
-	cache "github.com/pmylund/go-cache"
+	"github.com/pires/go-proxyproto"
+	"github.com/pmylund/go-cache"
 
 	"golang.org/x/net/http2"
 
@@ -148,6 +148,14 @@ func (m *proxyMux) setRouter(port int, protocol string, router *mux.Router) {
 	}
 }
 
+func (m *proxyMux) handle404(w http.ResponseWriter, r *http.Request) {
+	requestMeta := fmt.Sprintf("%s %s %s", r.Method, r.URL.Path, r.Proto)
+	log.WithField("request", requestMeta).Error(http.StatusText(http.StatusNotFound))
+
+	w.WriteHeader(http.StatusNotFound)
+	_, _ = fmt.Fprint(w, http.StatusText(http.StatusNotFound))
+}
+
 func (m *proxyMux) addTCPService(spec *APISpec, modifier *tcp.Modifier) {
 	hostname := spec.GlobalConfig.HostName
 	if spec.GlobalConfig.EnableCustomDomains {

gateway/proxy_muxer_test.go

@@ -12,6 +12,8 @@ import (
 	"sync/atomic"
 	"testing"
 
+	"github.com/TykTechnologies/tyk/test"
+
 	"github.com/TykTechnologies/tyk/config"
 )
 
@@ -289,3 +291,18 @@ func TestHTTP_custom_ports(t *testing.T) {
 		t.Errorf("expected %s to %s", echo, bs)
 	}
 }
+
+func TestHandle404(t *testing.T) {
+	g := StartTest()
+	defer g.Close()
+
+	BuildAndLoadAPI(func(spec *APISpec) {
+		spec.Proxy.ListenPath = "/existing"
+		spec.UseKeylessAccess = true
+	})
+
+	_, _ = g.Run(t, []test.TestCase{
+		{Path: "/existing", Code: http.StatusOK},
+		{Path: "/nonexisting", Code: http.StatusNotFound, BodyMatch: http.StatusText(http.StatusNotFound)},
+	}...)
+}