Descriptive error message in response to login error from login.gov

[raftmsohani]

Description:
Follow up on #1621, template error page is shown when AMS login fails with a description of the error. Similar change is needed for when login to login.gov fails.

Acceptance Criteria:

• Template error page which includes the HTTP error code and short description of error will be shown to the user when an error happens during login to cloud.gov
• Error is descriptive enough for troubleshooting
• HTTP error code is included
• Testing Checklist has been run and all tests pass
• README is updated, if necessary

Tasks:

• change api login to return template page in case of error (for reference see As a TDP user, I'd like to see a descriptive error message page if authentication source is unavailable. #1621)
• write unit tests as needed
• Run Testing Checklist and confirm all tests pass

[andrew-jameson]

PR merged and closed, ticket never entered sprint, accounting mistake, will close.
cc @vlasse86