Improves user lookup error message.

Closes #3188
Closes #3184

agent/util.go

@@ -61,12 +61,11 @@ func setFilePermissions(path string, user, group, mode string) error {
 		}
 
 		// Try looking up the user by name
-		if u, err := osuser.Lookup(user); err == nil {
-			uid, _ = strconv.Atoi(u.Uid)
-			goto GROUP
+		u, err := osuser.Lookup(user)
+		if err != nil {
+			return fmt.Errorf("failed to look up user %s: %v", user, err)
 		}
-
-		return fmt.Errorf("invalid user specified: %v", user)
+		uid, _ = strconv.Atoi(u.Uid)
 	}
 
 GROUP: