Improve error messages for invalid derivation names

src/libutil/error.hh

@@ -155,6 +155,7 @@ public:
         : err(e)
     { }
 
+    /** The error message without "error: " prefixed to it. */
     std::string message() {
         return err.msg.str();
     }

src/libutil/fmt.hh

@@ -111,6 +111,8 @@ std::ostream & operator<<(std::ostream & out, const Magenta<T> & y)
 /**
  * Values wrapped in this class are printed without coloring.
  *
+ * Specifically, the color is reset to normal before printing the value.
+ *
  * By default, arguments to `HintFmt` are printed in magenta (see `Magenta`).
  */
 template <class T>