[#535] Move error messages related to test to a separate page.

The idea is to write the common errors that might happen closer to where they're
more common, so you have the information nearby when you need it.

This works well for test errors. For install/upgrade, as we have two methods,
we can't simply add to the same page. I've changed the index so the common
errors page is under the Installation section. Only installation/upgrade
issues should be added there.