8297311: Improve exception message thrown by java.net.HostPortrange::toLowerCase(String s)

Reviewed-by: dfuchs, michaelm, rriggs

src/java.base/share/classes/java/net/HostPortrange.java

@@ -186,7 +186,8 @@ class HostPortrange {
                 }
                 sb.append((char)(c - CASE_DIFF));
             } else {
-                throw new IllegalArgumentException("Invalid characters in hostname");
+                final String message = String.format("Invalid character \\u%04x in hostname", (int) c);
+                throw new IllegalArgumentException(message);
             }
         }
         return sb == null ? s : sb.toString();