improve exception message in DateTimeFormatter (#731)

Co-authored-by: Stephen Colebourne <[email protected]>

src/main/java/org/joda/time/format/DateTimeFormat.java

@@ -679,7 +679,7 @@ private static boolean isNumericToken(String token) {
      */
     private static DateTimeFormatter createFormatterForPattern(String pattern) {
         if (pattern == null || pattern.length() == 0) {
-            throw new IllegalArgumentException("Invalid pattern specification");
+            throw new IllegalArgumentException("Invalid pattern specification. Pattern is null or empty.");
         }
         DateTimeFormatter formatter = cPatternCache.get(pattern);
         if (formatter == null) {