Add --warn-error option to dbt

[ayobamshy]

Background:

We have recently been tricked by dbt runs that didn't fail but gave a silent warning when models specified could not be found. Although this is inline with dbt documentation, adding the --warn-error option ensures all warnings are thrown as an unhandled exception.

In this PR:

• Added optional --warn-error switch in dbt command
• Some updates to README.md to reflect --warn-error added option and availability of DbtDocsGenerateOperator

[irfanhabib]

Thanks for the contribution @ayobamshy