Introduce Invalid* EC2 error codes

ec2_code attribute with valid EC2 error code was added to several
Nova exceptions to allow simple error code mapping in EC2 API.

Implements: blueprint ec2-error-codes

Change-Id: Ica5cdf4a9662b4b569ded1f1b1931b9a3cdaa9f3
Merge "Introduce Invalid* EC2 error codes"