enable more warnings
enable more warnings

* and add -Werror flag, disallowing warnings
* bump maven-plugin-api version to avoid evil warning
* note: -Xlint:processing would warn about
  org.apache.maven.plugins.annotations.* not being "claimed",
  this is not a problem it seems.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa