improve error message on invalid project name.

- issue

I want to redmine metrics,but following console log.

```
ERROR: Publisher hudson.plugins.redmine.RedmineMetricsPublisher aborted due to exception
java.lang.NullPointerException
        at hudson.plugins.redmine.RedmineMetricsCalculator.getVersionsString(RedmineMetricsCalculator.java:105)
        at hudson.plugins.redmine.RedmineMetricsCalculator.calc(RedmineMetricsCalculator.java:43)
        at hudson.plugins.redmine.RedmineMetricsPublisher.perform(RedmineMetricsPublisher.java:54)
        at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
        at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
        at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:779)
        at hudson.model.Build$BuildExecution.post2(Build.java:183)
        at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:726)
        at hudson.model.Run.execute(Run.java:1568)
        at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
        at hudson.model.ResourceController.execute(ResourceController.java:88)
        at hudson.model.Executor.run(Executor.java:236)
Finished: FAILURE
```

set project identifier, but Redmine plugin needs ProjectName.
I could not understand and confused.

- proposal

avoid NPE and include project name to exception message.