feat(8): adding style guide on error codes (#245)

Since error codes can be succinct when describing gRPC and
HTTP+JSON, we can just name them both in the AEPs.

Author: toumorokoshi

aep/general/0008/aep.md.j2

@@ -231,6 +231,10 @@ subdirectory); this ensures that the link works both on the AEP site, when
 viewing the Markdown file on GitHub, using the local development server, or a
 branch.
 
+### Error codes
+
+When referencing a error code, the prose **should** use the format `{google.api.error_code} / {http_status_code}`. For example, `OK / 200`.
+
 ## Rationale
 
 ### Designing for a broad set of clients