OPENDJ-1932 Code cleanup

* NewBaseDNPanel.java
** validateAutomaticallyGenerated(final Set<LocalizableMessage> errors)
*** Extract local constants
*** Add final keyword

* StatusGenericPanel.java
** checkIntValue(Collection<LocalizableMessage> errors, String stringValue,int minValue, int maxValue, LocalizableMessage errMsg)
*** Now return a boolean to allow client code to use conditionnal code after call
*** Don't throw runtime exception anymore (Thanks Jean-Noël for refactoring advices!)

* ExportLDIFPanel.java
** okClicked()
** Consequences of checkIntValue changes