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/services/rendering/interface.go

@@ -10,6 +10,7 @@ import (
 
 var ErrTimeout = errors.New("Timeout error. You can set timeout in seconds with &timeout url parameter")
 var ErrNoRenderer = errors.New("No renderer plugin found nor is an external render server configured")
+var ErrPhantomJSNotInstalled = errors.New("PhantomJS executable not found")
 
 type Opts struct {
 	Width    int