Error codes section of spec not up to date with error_message.dart in at_commons

[gkc]

Describe the bug
Error codes section of spec is not up to date with error_message.dart in at_commons.

• Multiple error codes have been added to error_message.dart which are not included in this doc

To Reproduce
See https://github.com/atsign-foundation/at_protocol/blob/trunk/specification/at_protocol_specification.md and compare with https://github.com/atsign-foundation/at_tools/blob/trunk/at_commons/lib/src/exception/error_message.dart

Expected behavior
All error codes defined in error_message.dart should be documented here

Additional context
While adding new code to at_commons's error_message.dart, I did a GitHub/atsign-foundation search for any mention of standard AT codes (I searched for AT0015) and found that this document is not up to date. Also found this one in the atsign.dev repo which also needs to be fixed I guess, although it seems odd to have two copies of effectively the same doc... @nickelskevin what is the intended relationship between the spec in this repo and the spec in the atsign.dev repo? should one just be a clone of the other?