Log stdlib error on failed exec.LookPath

The error returned by exec.LookPath was never surfaced to the user.
Without that detail, the user can't tell the difference between a
non-existent path and a permissions issue.

Additionally, when ExecuteCommand is an absolute path, we were still
attempting to prepend the CommandWorkingDirectory if the ExecuteCommand
was not found, which made it difficult to know which path the user
intended to execute.

This commit simplifies the logic to avoid multiple attempts with
ExecuteCommand is an absolute path and changes the error message from:

  error locating command: '/path/to/file'

to:

  error in exec: "/path/to/file": stat /path/to/file: no such file or directory
  error in exec: "/path/to/file": permission denied

Fixes #457

webhook.go

@@ -581,16 +581,16 @@ func handleHook(h *hook.Hook, rid string, headers, query, payload *map[string]in
 	var errors []error
 
 	// check the command exists
-	cmdPath, err := exec.LookPath(h.ExecuteCommand)
-	if err != nil {
-		// give a last chance, maybe is a relative path
-		relativeToCwd := filepath.Join(h.CommandWorkingDirectory, h.ExecuteCommand)
-		// check the command exists
-		cmdPath, err = exec.LookPath(relativeToCwd)
+	var lookpath string
+	if filepath.IsAbs(h.ExecuteCommand) || h.CommandWorkingDirectory == "" {
+		lookpath = h.ExecuteCommand
+	} else {
+		lookpath = filepath.Join(h.CommandWorkingDirectory, h.ExecuteCommand)
 	}
 
+	cmdPath, err := exec.LookPath(lookpath)
 	if err != nil {
-		log.Printf("[%s] error locating command: '%s'", rid, h.ExecuteCommand)
+		log.Printf("[%s] error in %s", rid, err)
 
 		// check if parameters specified in execute-command by mistake
 		if strings.IndexByte(h.ExecuteCommand, ' ') != -1 {