Better content header error and slight refactor of test helper
Merge pull request #279 from alphagov/clearer-content-type-error

Better content header error and slight refactor of test helper
Merge pull request #280 from alphagov/better-error-handling

Separate HTTP 500 error handling
Added test for new message text
Include incorrect token string in error message

https://www.pivotaltracker.com/story/show/69910594

[Delivers #69910594]