Mirroring: apply suggestions from code review (#16432)
Mirroring: apply suggestions from code review (#16432)

* Mirroring: apply suggestions from code review

* Fix error message

* Perform upstream mirroring if needed

* Fix function call
chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data