Bring better error message if compiler target is not set in Maven

[uschindler]

People get a error message that bundled signatures are not found. This is mostly caused by the fact that they forgt ot define the compiler target, so forbiddenapis cannot choose the right version of unsafe or deprecated signatures.

We should add a warning just before the bundled signatures are loaded, so they get the info that they hshould really define the compiler target (ideally through Maven property globally, so it applies also to compiler plugin).

[uschindler]

Oh this was already added:

if (bundledSignatures != null) {
  String targetVersion = getTargetVersion();
  if ("".equals(targetVersion)) targetVersion = null;
  if (targetVersion == null) {
    log.warn("The 'targetVersion' parameter or '${maven.compiler.target}' property is missing. " +
      "Trying to read bundled JDK signatures without compiler target. " +
      "You have to explicitely specify the version in the resource name.");
  }
  for (String bs : bundledSignatures) {
    log.info("Reading bundled API signatures: " + bs);
    checker.parseBundledSignatures(bs, targetVersion);
  }
}

[uschindler]

This was in issue #28