Always return a string in the 'message' field

Other errors return a formatted string from the message field. A
dashboard failing validation should follow this pattern so that
API clients can simply output the string rather than processing
it.
Update transaction and other links in api response

Spotlight expects the fields to be named `other` and `transaction`
rather than `other_links` and `transaction_link`
Merge pull request #220 from alphagov/bugfix-error-message-string

Always return a string in the 'message' field
Merge pull request #224 from alphagov/correct-links-output-in-dashboard-api

Update transaction and other links in api response