Restored 'Working with Validation Errors'

Somehow this got lost in my PRs before. I'm going to re-check all the
guides to make sure this didn't happen again elsewhere.
DebugExceptions `File.join` => `File.expand_path`
Merge pull request #8529 from schneems/schneems/debug-exceptions-extend-path

DebugExceptions `File.join` => `File.expand_path`
Add missing requires to core_ext/integer/time
Merge pull request #8532 from ptn/add_missing_require

Add missing requires to core_ext/integer/time