Removed unnecessary BasicErrorRenderer
Removed unnecessary BasicErrorRenderer

refs: https://github.com/TryGhost/Ghost/commit/4474ca1a1df67a6b678922c8681cf52fd662e60a
refs: https://github.com/TryGhost/Ghost/commit/0799f02e8034283e1097e01c1ebec32b74bdc4a1

The BasicErrorRenderer was created as a fallback for when we needed to not render templates, which is
chiefly when we're trying to render a 404 for an image. Using a template puts us at risk of an infinite 404 loop
if the missing image is referenced in the 404 template.

As of 0799f02e, the HTMLErrorRenderer no longer uses templates - instead we serve a very simple HTML page.
This can be used instead of the BasicErrorRenderer, as it results in a properly formatted error.
Even when sending responses in plain text, the content type is returned as HTML and therefore having an
unformatted error makes no sense - if we really need a non-html format I guess there should be no body at all.

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