Add a pointer to webfactory/exceptions-bundle which helps with development of custom error pages.
Point out the difference between error and exception pages.

Move section on testing error pages during development into a sidebar and make it shorter.
Update error_pages.rst
Use WebfactoryExceptionsBundle instead of webfactory/exceptions-bundle
feature #3577 Development of custom error pages is impractical if you need to set kernel.debug=false (mpdude)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3577).

Discussion
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Development of custom error pages is impractical if you need to set kernel.debug=false

Also see
https://github.com/symfony/symfony/issues/7446 and https://github.com/symfony/symfony/issues/1486

Commits
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6aa90e0 Use WebfactoryExceptionsBundle instead of webfactory/exceptions-bundle
2fee263 Update error_pages.rst
435019c Point out the difference between error and exception pages.
2194607 Add a pointer to webfactory/exceptions-bundle which helps with development of custom error pages.