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/phantomjs.go

@@ -24,6 +24,11 @@ func (rs *RenderingService) renderViaPhantomJS(ctx context.Context, opts Opts) (
 
 	url := rs.getURL(opts.Path)
 	binPath, _ := filepath.Abs(filepath.Join(rs.Cfg.PhantomDir, executable))
+	if _, err := os.Stat(binPath); os.IsNotExist(err) {
+		rs.log.Error("executable not found", "executable", binPath)
+		return nil, ErrPhantomJSNotInstalled
+	}
+
 	scriptPath, _ := filepath.Abs(filepath.Join(rs.Cfg.PhantomDir, "render.js"))
 	pngPath := rs.getFilePathForNewImage()