Throw an WatcherSettingsException when parsing an invalid hourly schedule.

```
   "schedule" : {
       "hourly" :  [0]
}
```
Was throwing an NPE now it gives a WatcherSettingsException.

Fixes: elastic/elasticsearch#419

Original commit: elastic/x-pack-elasticsearch@f334712f96

src/main/java/org/elasticsearch/watcher/WatcherSettingsException.java

@@ -10,11 +10,12 @@ package org.elasticsearch.watcher;
  */
 public class WatcherSettingsException extends WatcherException {
 
-    public WatcherSettingsException(String msg, Throwable cause) {
-        super(msg, cause);
+    public WatcherSettingsException(String msg, Object... args) {
+        super(msg, args);
     }
 
-    public WatcherSettingsException(String msg) {
-        super(msg);
+    public WatcherSettingsException(String msg, Throwable cause, Object... args) {
+        super(msg, cause, args);
     }
+
 }

src/main/java/org/elasticsearch/watcher/trigger/schedule/CronSchedule.java

@@ -78,7 +78,7 @@ public class CronSchedule extends CronnableSchedule {
         private String expression;
 
         public ValidationException(String expression, Cron.ParseException cause) {
-            super("invalid cron expression [" + expression + "]. " + cause.getMessage());
+            super("invalid cron expression [{}]", cause, expression);
             this.expression = expression;
         }
     }

src/main/java/org/elasticsearch/watcher/trigger/schedule/HourlySchedule.java

@@ -99,6 +99,8 @@ public class HourlySchedule extends CronnableSchedule {
             while ((token = parser.nextToken()) != XContentParser.Token.END_OBJECT) {
                 if (token == XContentParser.Token.FIELD_NAME) {
                     currentFieldName = parser.currentName();
+                } else if (currentFieldName == null) {
+                    throw new WatcherSettingsException("could not parse [{}] schedule. unexpected token [{}]", TYPE, token);
                 } else if (MINUTE_FIELD.match(currentFieldName)) {
                     if (token.isValue()) {
                         try {