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:
Not started
Approver:
Sean McGinnis
Priority:
Medium
Drafter:
Cao ShuFeng
Direction:
Approved
Assignee:
Cao ShuFeng
Definition:
Approved
Series goal:
None
Implementation:
Unknown
Milestone target:
None

Related branches

Sprints

Whiteboard

Gerrit topic: https://review.openstack.org/#q,topic:bp/error-codes-support,n,z

Addressed by: https://review.openstack.org/293306
    Enhance error messages for API user

Addressed by: https://review.openstack.org/296304
    [WIP] ehanche error message

Addressed by: https://review.openstack.org/346715
    Open Ocata for specs

(?)

Work Items

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