fix: Make CONNECTION_CLOSED_ERROR a static property

Fixes #460

Author: kadler

lib/Connection.js

@@ -3,7 +3,7 @@ const { Cursor }    = require('./Cursor');
 
 class Connection {
 
-  CONNECTION_CLOSED_ERROR = 'Connection has already been closed!';
+  static CONNECTION_CLOSED_ERROR = 'Connection has already been closed!';
 
   /**
    * An open connection to the database made through an ODBC driver
@@ -26,7 +26,7 @@ class Connection {
   get autocommit() {
     if (!this.odbcConnection)
     {
-      throw new Error(CONNECTION_CLOSED_ERROR);
+      throw new Error(Connection.CONNECTION_CLOSED_ERROR);
     }
     return this.odbcConnection.autocommit;
   }