Allow public access to the dev/health url and change response to 500

[stojg]

According to the documentation, the dev/health route should be publically accessible, this PR removes the current permission check.

In addition to this, the default return error code is 500 (server error) instead of 404 (which is regarded as a client error).

[helpfulrobot]

@stojg, thanks for your PR! By analyzing the blame information on this pull request, I identified @sminnee, @assertchris and @halkyon to be potential reviewers

[dhensby]

It would be good for the old url to forward to the new one, otherwise this is a pretty significant change and would need to be in the next major release.

[stojg]

I would prefer to keep the old URL and allow open access to the dev/health endpoint since that is the documented behaviour but was changed with the changes in https://github.com/silverstripe-labs/silverstripe-environmentcheck/pull/17/files but @tractorcow want everything under dev/ to be protected.

[tractorcow]

You need to add $usesDatabase = true to your tests now.

See silverstripe/silverstripe-framework#5557 (comment) for reference.

[tractorcow]

Just merge #35 after tests pass, then rebase this. :D