docs: add a common_errors file to hopefully help avoid debugging the …

[goapunk]

…same problems over and over again

Tasks

• [ ] PR name contains story or task reference
• [x] Documentation (docs and inline)
• [ ] Tests (including n+1 and django_assert_num_queries where applicable)
• [ ] Changelog

[m4ra]

@hom3mad3 have a look at this as it about frontend testing errors