Upgrade tasks fail if a builder plugin is missing.

2008-04-18 16:21:40,803 INFO [main] [UpgradeManagerImpl] Completed upgrade task 602 successfully.
2008-04-18 16:21:40,815 INFO [main] [UpgradeManagerImpl] ---------------------------------------------------------------------------------
2008-04-18 16:21:40,815 INFO [main] [UpgradeManagerImpl] Running upgrade task 603 : Add a vcsRevisionKey field for Build result Summaries.
2008-04-18 16:21:40,816 INFO [main] [UpgradeManagerImpl] ---------------------------------------------------------------------------------
2008-04-18 16:21:40,908 INFO [main] [UpgradeTask603VcsKeyForBuild] Updating build plan 'sdfa - Default'
2008-04-18 16:21:40,909 INFO [main] [DefaultBuildDefinitionManager] Getting Build Configuration for 'EFH45-DEF'.
2008-04-18 16:21:41,015 ERROR [main] [UpgradeManagerImpl] java.lang.IllegalArgumentException: Could not find builder with key 'com.atlassian.bamboo.plugin.builder.nant:nant'
java.lang.IllegalArgumentException: Could not find builder with key 'com.atlassian.bamboo.plugin.builder.nant:nant'
	at com.atlassian.bamboo.builder.DefaultBuilderManager.getBuilder(DefaultBuilderManager.java:131)
	at com.atlassian.bamboo.fieldvalue.BuildDefinitionConverter.toObject(BuildDefinitionConverter.java:172)
	at com.atlassian.bamboo.build.DefaultBuildDefinitionManager.getBuildDefinitionFromBuild(DefaultBuildDefinitionManager.java:73)
	at com.atlassian.bamboo.build.DefaultBuildDefinitionManager.getBuildDefinition(DefaultBuildDefinitionManager.java:31)
	at com.atlassian.bamboo.build.DefaultBuild.getBuildDefinition(DefaultBuild.java:411)
	at com.atlassian.bamboo.upgrade.tasks.UpgradeTask603VcsKeyForBuild.doUpgrade(UpgradeTask603VcsKeyForBuild.java:58)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:304)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149)
	at org.springframework.orm.hibernate.HibernateInterceptor.invoke(HibernateInterceptor.java:117)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
	at $Proxy36.doUpgrade(Unknown Source)
	at com.atlassian.bamboo.upgrade.UpgradeManagerImpl.runUpgradeTask(UpgradeManagerImpl.java:122)
	at com.atlassian.bamboo.upgrade.UpgradeManagerImpl.doUpgrade(UpgradeManagerImpl.java:88)
	at com.atlassian.bamboo.upgrade.UpgradeLauncher.upgradeAndStartBamboo(UpgradeLauncher.java:104)
	at com.atlassian.bamboo.upgrade.UpgradeLauncher.contextInitialized(UpgradeLauncher.java:37)
	at org.mortbay.jetty.handler.ContextHandler.startContext(ContextHandler.java:530)
	at org.mortbay.jetty.servlet.Context.startContext(Context.java:135)
	at org.mortbay.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1218)
	at org.mortbay.jetty.handler.ContextHandler.doStart(ContextHandler.java:500)
	at org.mortbay.jetty.webapp.WebAppContext.doStart(WebAppContext.java:448)
	at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:40)
	at org.mortbay.jetty.handler.HandlerWrapper.doStart(HandlerWrapper.java:117)
	at org.mortbay.jetty.Server.doStart(Server.java:217)
	at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:40)
	at com.atlassian.bamboo.server.Server.main(Server.java:60)
2008-04-18 16:21:41,017 INFO [main] [UpgradeManagerImpl] Completed upgrade task 603 with errors.