Fixed NPE when LOTL loading fails

Created by: rvillido

This modification fixes NullPointerException when LOTL loading fails and throws a specific DSS exception so such situations could be handled gracefully.

01.12.2015 14:26:06.204 WARN  [pool-197-thread-7] [e.e.esig.dss.tsl.service.TSLLoader.call:58] - Unable to load 'http://references.modernisation.gouv.fr/sites/default/files/TSL-FR.xml' : org.apache.http.conn.HttpHostConnectException: Connect to references.modernisation.gouv.fr:80 [references.modernisation.gouv.fr/217.74.104.3] failed: Operation timed out
01.12.2015 14:26:06.205 ERROR [main] [e.e.e.d.tsl.service.TSLValidationJob.analyzeCountryPointers:243] - Unable to load/parse TSL : null
  java.lang.NullPointerException: null
    at java.security.MessageDigest.update(MessageDigest.java:335)
    at java.security.MessageDigest.digest(MessageDigest.java:410)
    at eu.europa.esig.dss.DSSUtils.digest(DSSUtils.java:587)
    at eu.europa.esig.dss.tsl.service.TSLRepository.getSHA256(TSLRepository.java:270)
    at eu.europa.esig.dss.tsl.service.TSLRepository.storeInCache(TSLRepository.java:209)
    at eu.europa.esig.dss.tsl.service.TSLValidationJob.analyzeCountryPointers(TSLValidationJob.java:226)
    at eu.europa.esig.dss.tsl.service.TSLValidationJob.refresh(TSLValidationJob.java:203)