Display a more intelligent error message if the user runs a locus traversal across an unmapped reads file.

git-svn-id: file:///humgen/gsa-scr1/gsa-engineering/svn_contents/trunk@1238 348d0f76-0448-11de-a6fe-93d51630548a

java/src/org/broadinstitute/sting/gatk/datasources/simpleDataSources/SAMDataSource.java

@@ -161,6 +161,8 @@ public class SAMDataSource implements SimpleDataSource {
      * @return an iterator for that region
      */
     private StingSAMIterator seekLocus( GenomeLoc location ) throws SimpleDataSourceLoadException {
+        if( getHeader().getSequenceDictionary().getSequences().size() == 0 )
+            throw new StingException("Unable to seek to the given locus; reads data source has no alignment information.");
         return createIterator( new MappedStreamSegment(location) );
     }