#199 no clear error message on missing public key

[Fred Kastl]

if a referenced column is not a public key, the ant task terminates with the exception below.
In this case a clear error message would be nice.

I'm using the current cvs version.

## Exception ###

[middlegen] No <table> elements specified. Reading all tables. This might take a while...
[middlegen] java.lang.NullPointerException
[middlegen] at middlegen.plugins.hibernate.HibernateRelationshipRole.init(HibernateRelationshipRole.java:259)
[middlegen] at middlegen.Plugin.decorateAll(Plugin.java:387)
[middlegen] at middlegen.Middlegen.decorateAll(Middlegen.java:312)
[middlegen] at middlegen.MiddlegenTask.execute(MiddlegenTask.java:424)
[middlegen] at org.apache.tools.ant.UnknownElement.execute(UnknownElement.java:275)
[middlegen] at org.apache.tools.ant.Task.perform(Task.java:364)
[middlegen] at org.apache.tools.ant.Target.execute(Target.java:341)
[middlegen] at org.apache.tools.ant.Target.performTasks(Target.java:369)
[middlegen] at org.apache.tools.ant.Project.executeSortedTargets(Project.java:1216)
[middlegen] at org.apache.tools.ant.Project.executeTarget(Project.java:1185)
[middlegen] at org.apache.tools.ant.helper.DefaultExecutor.executeTargets(DefaultExecutor.java:40)
[middlegen] at org.eclipse.ant.internal.ui.antsupport.EclipseDefaultExecutor.executeTargets(EclipseDefaultExecutor.java:32)
[middlegen] at org.apache.tools.ant.Project.executeTargets(Project.java:1068)
[middlegen] at org.eclipse.ant.internal.ui.antsupport.InternalAntRunner.run(InternalAntRunner.java:423)
[middlegen] at org.eclipse.ant.internal.ui.antsupport.InternalAntRunner.main(InternalAntRunner.java:137)