Throw exception if sequence generator not found
Start porting Behat 2 into new & shinny Behat 3
Split contexts into smaller parts, more aligned with bundles
Adjust code after review:
- Remove left-overs from Behat v2
- Update `composer.json` & `composer.lock` files
Merge pull request #1500 from umpirsky/feature/sequence-error-handling

Throw exception if sequence generator not found