add error codes to HTTP error responses

Registered by Cao ShuFeng

According to this spec:
http://specs.openstack.org/openstack/api-wg/guidelines/errors.html
When error happens, a user-friendly error message with error code and help href shall be returned to users.
With error codes, user may get a better pool of web pages from google.

Blueprint information

Status:
Complete
Approver:
None
Priority:
Undefined
Drafter:
Cao ShuFeng
Direction:
Needs approval
Assignee:
Cao ShuFeng
Definition:
Obsolete
Series goal:
None
Implementation:
Unknown
Milestone target:
None
Completed by
Armando Migliaccio

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