make error readable for user - not only confusing stacktrace

I would expect with a yaml:

```
jenkins:
systemMessages: "Jenkins configured automatically by Jenkins Configuration as Code Plugin\n\n"
```

I'll get a warning from dryrun saying the file is not applicable - it's `systemMessage`, not a plural `systemMessages`. Is this how dryrun should work? :)

Instead I get this stacktrace:

```
io.jenkins.plugins.casc.ConfiguratorException: Invalid configuration elements for type class jenkins.model.Jenkins : systemMessages
at io.jenkins.plugins.casc.BaseConfigurator.handleUnknown(BaseConfigurator.java:355)
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:345)
at io.jenkins.plugins.casc.BaseConfigurator.check(BaseConfigurator.java:265)
at io.jenkins.plugins.casc.ConfigurationAsCode.lambda$checkWith$8(ConfigurationAsCode.java:641)
at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:606)
Caused: io.jenkins.plugins.casc.ConfiguratorException: jenkins: error configuring 'jenkins' with class io.jenkins.plugins.casc.core.JenkinsConfigurator configurator
at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:612)
at io.jenkins.plugins.casc.ConfigurationAsCode.checkWith(ConfigurationAsCode.java:641)
at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:628)
at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:540)
at io.jenkins.plugins.casc.ConfigurationAsCode.configure(ConfigurationAsCode.java:270)
at io.jenkins.plugins.casc.ConfigurationAsCode.doReload(ConfigurationAsCode.java:151)
at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627)
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:343)
at org.kohsuke.stapler.interceptor.RequirePOST$Processor.invoke(RequirePOST.java:77)
at org.kohsuke.stapler.PreInvokeInterceptedFunction.invoke(PreInvokeInterceptedFunction.java:26)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:184)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:117)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:129)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:58)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845)
at org.kohsuke.stapler.MetaClass$10.dispatch(MetaClass.java:374)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:841)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1650)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:154)
at jenkins.metrics.impl.MetricsFilter.doFilter(MetricsFilter.java:125)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:157)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:64)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:90)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1637)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:533)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:524)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:190)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1595)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:188)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1253)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:168)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:473)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1564)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:166)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1155)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:219)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:126)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
at org.eclipse.jetty.server.Server.handle(Server.java:564)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:317)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:251)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:279)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:110)
at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:124)
at org.eclipse.jetty.util.thread.Invocable.invokePreferred(Invocable.java:128)
at org.eclipse.jetty.util.thread.Invocable$InvocableExecutor.invoke(Invocable.java:222)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:294)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:199)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:672)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:590)
at java.lang.Thread.run(Thread.java:745)
```

cc @ndeloof