Add better error message when chromium isn't found

When the chromium isn't found, we say: "no command", which is confusing
to most users. This new error message tells the user that they have to
install a Chromium on their system to run browser tests.

chromium/browser_type.go

@@ -241,9 +241,12 @@ func (b *BrowserType) allocate(
 	}
 
 	path := opts.ExecutablePath
-	if path == "" {
+	if strings.TrimSpace(path) == "" {
 		path = b.ExecutablePath()
 	}
+	if path == "" { // still empty?
+		return nil, fmt.Errorf("chromium is not installed on this system")
+	}
 
 	return common.NewLocalBrowserProcess(bProcCtx, path, args, dataDir, bProcCtxCancel, logger) //nolint: wrapcheck
 }