Add error code reference section (#3820)

* add error code page and update TOC

* removed obsolute and not-yet-available

Author: MongoCaleb

source/reference.txt

@@ -38,6 +38,9 @@ Reference
 :ref:`server-exit-codes`
    Details the codes and statuses that MongoDB returns when exiting.
 
+:ref:`server-error-codes`
+   Details the error codes that MongoDB returns.
+
 :ref:`explain-results`
    Documentation on information returned from explain operations.
 
@@ -113,6 +116,7 @@ Reference
    /reference/default-mongodb-port
    /reference/mongodb-defaults
    /reference/exit-codes
+   /reference/error-codes
    /reference/explain-results
    /reference/glossary
    /reference/log-messages