Adds per-server/location error-catch functionality to nginx template
Adds documentation
Reduces template duplication with helper function for CUSTOM_ERRORS data
Updates documentation
Adds e2e test for customerrors
Removes AllCustomHTTPErrors, replaces with template function with deduplication and adds e2e test of deduplication
Fixes copy-paste error in test, adds additional test cases
Reverts noop change in controller.go (unused now)
