Move common error codes to errcode package

Several error codes are generally useful but tied to the v2 specification
definitions. This change moves these error code definitions into the common
package for use by the health package, which is not tied to the v2 API.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/spec/api.md

@@ -2249,7 +2249,7 @@ The following headers will be returned with the response:
 
 |Name|Description|
 |----|-----------|
-|`Content-Length`|Zero|
+|`Content-Length`|0|
 |`Docker-Content-Digest`|Digest of the targeted content for the request.|