Add more verbose error message if 2FA required

The previous message was just "2FA required." Let's do more here, and say it's an authentication failure. Explain why, and let's see if we can't also garden in a link to the docs for how to do this.

Bonus points:

• can we link to https://docs.gitlab.com/ee/user/profile/account/two_factor_authentication.html#enable-two-factor-authentication ?
• can this message be made translatable?

Related

• Closes Polish error message when 2fa is required (#357870 - closed)
• Identified as a post-merge followup in !84151 (comment 900909451) (!84151 (comment 900909451)+)