Only raise DelegationError if it's is the source of the exception

This fixes situations where nested NoMethodError exceptions are masked
by delegations. This would cause confusion especially where there was a
problem in the Rails booting process because of a delegation in the
routes reloading code.

Fixes #10559
Add missing require
Fix a grammatical error/typo in Active Record Associations guide.
Merge pull request #11400 from dpdawson/master

Fix a grammatical error/typo in Active Record Associations guide. [ci skip]
Documment config.log_formatter added on 51aeae91

[ci skip]
Improve Active View Overview guide. [ci skip]
Merge pull request #11401 from dpdawson/master

Improved Active View Overview guide. [ci skip]
move `MissingHelperError` out of the `ClassMethods` module.
add test-case to verify `error.path` when a helper was not found.

This verifies the code added with:
https://github.com/rails/rails/commit/99c9d18601539c7e7e87f26bb047add1f93072af