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
Merge branch '2.6' into 2.7

* 2.6:
  Add preview error route to dev firewall
Issue #758 Modified default route for AppBundle
bug #762 Issue #758 Modified default route for AppBundle (Pierstoval)

This PR was merged into the 2.3 branch.

Discussion
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Issue #758 Modified default route for AppBundle

Commits
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223e6c8 Issue #758 Modified default route for AppBundle
Merge branch '2.3' into 2.5

* 2.3:
  Issue #758 Modified default route for AppBundle
Merge branch '2.5' into 2.6

* 2.5:
  Issue #758 Modified default route for AppBundle
Merge branch '2.6' into 2.7

* 2.6:
  Issue #758 Modified default route for AppBundle
Merge branch '2.7'

* 2.7:
  Issue #758 Modified default route for AppBundle
  Add preview error route to dev firewall