Add preview error route to dev firewall

Add the `_error` route to preview error pages to the dev firewall, so you don't have to be authenticated to preview error pages
bug #759 Add preview error route to dev firewall (pierredup)

This PR was submitted for the 2.7 branch but it was merged into the 2.6 branch instead (closes #759).

Discussion
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Add preview error route to dev firewall

Add the `_error` route to preview error pages to the dev firewall, so you don't have to be authenticated to preview error pages

Commits
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63cb670 Add preview error route to dev firewall