Show better error when PAT can't authenticate to a private server

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Bundler does not help a lot newbie users trying to configure a private gem server.

What is your fix for the problem, implemented in this PR?

Hopefully give better error messages.

Before:

Fetching gem metadata from https://rubygems.org/........
Fetching source index from https://rubygems.pkg.github.com/my-org/

Bad username or password for https://x-access-token@rubygems.pkg.github.com/my-org/.
Please double-check your credentials and correct them.

After:

Fetching gem metadata from https://rubygems.org/........
Fetching source index from https://rubygems.pkg.github.com/my-org/

Access token could not be authenticated for https://x-access-token@rubygems.pkg.github.com/my-org/.
Make sure it's valid and has the necessary scopes configured.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags