Adding "Address not found" generic page.

Makefile

@@ -445,8 +445,10 @@ release: require-version require-s3cmd require-gh-token require-wget require-rub
 
 update-resources:
 	@(which go-bindata >/dev/null) || (echo 'Missing command "go-bindata". Sett https://github.com/jteeuwen/go-bindata.' && exit 1) && \
-	go-bindata -nomemcopy -nocompress -pkg main -o src/github.com/getlantern/flashlight/resources.go -prefix src/github.com/getlantern/flashlight/ \
-	src/github.com/getlantern/flashlight/icons src/github.com/getlantern/flashlight/status_pages
+	go-bindata -nomemcopy -nocompress -pkg main -o src/github.com/getlantern/flashlight/resources.go -prefix \
+	src/github.com/getlantern/flashlight/ src/github.com/getlantern/flashlight/icons && \
+	go-bindata -nomemcopy -nocompress -pkg status -o src/github.com/getlantern/flashlight/status/resources.go -prefix \
+	src/github.com/getlantern/flashlight/status_pages src/github.com/getlantern/flashlight/status_pages
 
 create-tag: require-version
 	@git tag -a "$$VERSION" -f --annotate -m"Tagged $$VERSION" && \

src/github.com/getlantern/flashlight/client/reverseproxy.go

@@ -1,6 +1,8 @@
 package client
 
 import (
+	"bytes"
+	"io/ioutil"
 	"net/http"
 	"net/http/httputil"
 	"runtime"
@@ -9,6 +11,7 @@ import (
 	"github.com/getlantern/balancer"
 	"github.com/getlantern/detour"
 	"github.com/getlantern/flashlight/proxy"
+	"github.com/getlantern/flashlight/status"
 )
 
 // getReverseProxy waits for a message from client.rpCh to arrive and then it
@@ -52,9 +55,9 @@ func (client *Client) initReverseProxy(bal *balancer.Balancer, dumpHeaders bool)
 		Director: func(req *http.Request) {
 			// do nothing
 		},
-		Transport: withDumpHeaders(
-			dumpHeaders,
-			transport),
+		Transport: &errorRewritingRoundTripper{
+			withDumpHeaders(dumpHeaders, transport),
+		},
 		// Set a FlushInterval to prevent overly aggressive buffering of
 		// responses, which helps keep memory usage down
 		FlushInterval: 250 * time.Millisecond,
@@ -102,3 +105,32 @@ func (rt *headerDumpingRoundTripper) RoundTrip(req *http.Request) (resp *http.Re
 	}
 	return
 }
+
+// The errorRewritingRoundTripper writes creates an special *http.Response when
+// the roundtripper fails for some reason.
+type errorRewritingRoundTripper struct {
+	orig http.RoundTripper
+}
+
+func (er *errorRewritingRoundTripper) RoundTrip(req *http.Request) (resp *http.Response, err error) {
+	res, err := er.orig.RoundTrip(req)
+	if err != nil {
+
+		// It is likely we will have lots of different errors to handle but for now
+		// we will only return a CannotFindServer error.  This prevents the user
+		// from getting just a blank screen.
+		htmlerr, err := status.CannotFindServer(req.Host, err)
+
+		if err != nil {
+			log.Debugf("Got error while generating status page: %q", err)
+		}
+
+		res = &http.Response{
+			Body: ioutil.NopCloser(bytes.NewBuffer(htmlerr)),
+		}
+
+		res.StatusCode = http.StatusServiceUnavailable
+		return res, nil
+	}
+	return res, err
+}

src/github.com/getlantern/flashlight/status/status.go

@@ -0,0 +1,56 @@
+package status
+
+import (
+	"bytes"
+	"errors"
+	"html/template"
+	"strings"
+)
+
+type cannotFindServerT struct {
+	ServerName   string
+	ErrorMessage string
+}
+
+func normalizeError(err error) string {
+	if err != nil {
+		content := strings.SplitN(strings.TrimSpace(err.Error()), "\n", 2)
+		return strings.TrimSpace(content[0])
+	}
+	return ""
+}
+
+// CannotFindServer creates and returns a generic "cannot find server" error.
+func CannotFindServer(server string, errMessage error) ([]byte, error) {
+	var err error
+	var buf []byte
+	var tmpl *template.Template
+
+	if errMessage == nil {
+		errMessage = errors.New("Unknown error.")
+	}
+
+	buf, err = Asset("generic_error.html")
+
+	if err != nil {
+		return nil, err
+	}
+
+	tmpl, err = template.New("status_error").Parse(string(buf))
+	if err != nil {
+		return nil, err
+	}
+
+	data := cannotFindServerT{
+		ServerName:   server,
+		ErrorMessage: normalizeError(errMessage),
+	}
+
+	out := bytes.NewBuffer(nil)
+
+	if err = tmpl.Execute(out, data); err != nil {
+		return nil, err
+	}
+
+	return out.Bytes(), nil
+}