fix(i/t/tor.go): show correct command line (#673)

While there, ensure that we print a warning if we cannot find
the correct tor binary.

Work part of https://github.com/ooni/probe/issues/1917.

internal/tunnel/tordesktop.go

@@ -4,16 +4,22 @@ package tunnel
 
 // This file implements our strategy for running tor on desktop.
 
-import "github.com/cretz/bine/tor"
+import (
+	"strings"
+
+	"github.com/cretz/bine/tor"
+)
 
 // getTorStartConf in this configuration uses torExePath to get a
 // suitable tor binary and then executes it.
 func getTorStartConf(config *Config, dataDir string, extraArgs []string) (*tor.StartConf, error) {
 	exePath, err := config.torBinary()
 	if err != nil {
+		config.logger().Warnf("cannot find tor binary: %s", err.Error())
 		return nil, err
 	}
-	config.logger().Infof("tunnel: tor: exec binary: %s", exePath)
+	config.logger().Infof("tunnel: tor: exec: %s %s %s", exePath,
+		dataDir, strings.Join(extraArgs, " "))
 	return &tor.StartConf{
 		ExePath:   exePath,
 		DataDir:   dataDir,