StmtMetadataLoader::collectPolicyTables() should handle FailedLoadLocalTable without NPE

If table loading fails, then it result in a FailedLoadLocalTable, which uses this constructor for LocalTable:

  protected LocalTable(LocalDb db, String tblName) {
    this.db_ = Preconditions.checkNotNull(db);
    this.name_ = tblName;
    this.ref_ = null;
    this.msTable_ = null;
    this.cols_ = null;
    this.tableStats_ = null;
  }

Since cols_ and other fields are null, the code in StmtMetadataLoader::collectPolicyTables(), which calls LocalTable::getColumnsInHiveOrder() will throw a NullPointerException when trying to access cols_ in getNonClusteringColumns(). 

This will fail the query with a NullPointerException, even though the underlying problem came from failing to load the table metadata. We should handle this to avoid the NullPointerException. That may happen as part of StmtMetadataLoader::collectPolicyTables() or StmtMetadataLoader::getMissingTables().