enhance error message if phantomjs executable is not found

if arm build, explain that phantomjs is not included by default in arm
builds. If not explain that phantom js isn't installed correctly

pkg/api/render.go

@@ -3,7 +3,9 @@ package api
 import (
 	"fmt"
 	"net/http"
+	"runtime"
 	"strconv"
+	"strings"
 	"time"
 
 	m "github.com/grafana/grafana/pkg/models"
@@ -55,6 +57,15 @@ func (hs *HTTPServer) RenderToPng(c *m.ReqContext) {
 		return
 	}
 
+	if err != nil && err == rendering.ErrPhantomJSNotInstalled {
+		if strings.HasPrefix(runtime.GOARCH, "arm") {
+			c.Handle(500, "Rendering failed - PhantomJS isn't included in arm build per default", err)
+		} else {
+			c.Handle(500, "Rendering failed - PhantomJS isn't installed correctly", err)
+		}
+		return
+	}
+
 	if err != nil {
 		c.Handle(500, "Rendering failed.", err)
 		return