Check for iteration errors in PurgeMissing

Not checking for iterator errors was hiding problems with pipeline
result size limits being exceeded when PurgeMissing was being used to
recover a production database.

txn/txn.go

@@ -417,6 +417,9 @@ func (r *Runner) PurgeMissing(collections ...string) error {
 				return fmt.Errorf("error purging missing transaction %s: %v", txnId.Hex(), err)
 			}
 		}
+		if err := iter.Close(); err != nil {
+			return fmt.Errorf("transaction queue iteration error for collection %s: %v", collection, err)
+		}
 		colls[collection] = true
 	}
 
@@ -442,6 +445,9 @@ func (r *Runner) PurgeMissing(collections ...string) error {
 			return fmt.Errorf("error purging missing transaction %s: %v", txnId.Hex(), err)
 		}
 	}
+	if err := iter.Close(); err != nil {
+		return fmt.Errorf("transaction stash iteration error: %v", err)
+	}
 
 	return nil
 }