title: Error in Sonar while executing Findbug rules
body: <p>I'm getting following error after configuring my sonar project with findbugs. Please help me out.</p>
  
  <pre><code>BUILD FAILED
  17-May-2013 11:18:15    /home/build/bamboo-home/xml-data/build-dir/EZIO-EMA-JOB1/build.xml:247: org.sonar.api.utils.SonarException: Can not execute Findbugs
  17-May-2013 11:18:15            at org.sonar.plugins.findbugs.FindbugsExecutor.execute(FindbugsExecutor.java:120)
  17-May-2013 11:18:15            at org.sonar.plugins.findbugs.FindbugsSensor.analyse(FindbugsSensor.java:58)
  17-May-2013 11:18:15            at org.sonar.batch.phases.SensorsExecutor.execute(SensorsExecutor.java:64)
  17-May-2013 11:18:15            at org.sonar.batch.phases.Phases.execute(Phases.java:93)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.ProjectModule.doStart(ProjectModule.java:143)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.Module.start(Module.java:89)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.BatchModule.analyze(BatchModule.java:110)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.BatchModule.doStart(BatchModule.java:100)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.Module.start(Module.java:89)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.BootstrapModule.doStart(BootstrapModule.java:96)
  17-May-2013 11:18:15            at org.sonar.batch.bootstrap.Module.start(Module.java:89)
  17-May-2013 11:18:15            at org.sonar.batch.Batch.execute(Batch.java:74)
  17-May-2013 11:18:15            at org.sonar.ant.Launcher.execute(Launcher.java:70)
  17-May-2013 11:18:15            at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  17-May-2013 11:18:15            at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
  17-May-2013 11:18:15            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  17-May-2013 11:18:15            at java.lang.reflect.Method.invoke(Method.java:597)
  17-May-2013 11:18:15            at org.sonar.ant.SonarTask.delegateExecution(SonarTask.java:202)
  17-May-2013 11:18:15            at org.sonar.ant.SonarTask.execute(SonarTask.java:186)
  17-May-2013 11:18:15            at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:291)
  17-May-2013 11:18:15            at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
  17-May-2013 11:18:15            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  17-May-2013 11:18:15            at java.lang.reflect.Method.invoke(Method.java:597)
  17-May-2013 11:18:15            at org.apache.tools.ant.dispatch.DispatchUtils.execute(DispatchUtils.java:106)
  17-May-2013 11:18:15            at org.apache.tools.ant.Task.perform(Task.java:348)
  17-May-2013 11:18:15            at org.apache.tools.ant.Target.execute(Target.java:390)
  17-May-2013 11:18:15            at org.apache.tools.ant.Target.performTasks(Target.java:411)
  17-May-2013 11:18:15            at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1360)
  17-May-2013 11:18:15            at org.apache.tools.ant.Project.executeTarget(Project.java:1329)
  17-May-2013 11:18:15            at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:41)
  17-May-2013 11:18:15            at org.apache.tools.ant.Project.executeTargets(Project.java:1212)
  17-May-2013 11:18:15            at org.apache.tools.ant.Main.runBuild(Main.java:801)
  17-May-2013 11:18:15            at org.apache.tools.ant.Main.startAnt(Main.java:218)
  17-May-2013 11:18:15            at org.apache.tools.ant.launch.Launcher.run(Launcher.java:280)
  17-May-2013 11:18:15            at org.apache.tools.ant.launch.Launcher.main(Launcher.java:109)
  17-May-2013 11:18:15    Caused by: org.sonar.api.utils.SonarException: Findbugs needs sources to be compiled. Please build project before executing sonar and check the location of compiled classes.
  17-May-2013 11:18:15            at org.sonar.plugins.findbugs.FindbugsConfiguration.getFindbugsProject(FindbugsConfiguration.java:66)
  17-May-2013 11:18:15            at org.sonar.plugins.findbugs.FindbugsExecutor.execute(FindbugsExecutor.java:74)
  17-May-2013 11:18:15            ... 34 more
  </code></pre>