Update the README
Fall back to path for write api errors

Use the request path instead of the bucket name for errors if the bucket
name has not been set.

@maxfliri
@robyoung
Add tests for exception handling

As a result of adding tests add better defaults for the response status and error message.
Merge pull request #86 from alphagov/write-api-get-error

Fall back to path for write api errors
Merge branch 'master' into signonotron2-integration
Refactor to use flash messages consistently

@gtrogers
@pbadenski