add some error handling for item size calculation in LRUCache
add some error handling for item size calculation in LRUCache

nathandunn/jbrowse,GMOD/jbrowse,GMOD/jbrowse,GMOD/jbrowse,nathandunn/jbrowse,nathandunn/jbrowse,GMOD/jbrowse,nathandunn/jbrowse,nathandunn/jbrowse,GMOD/jbrowse