Prevent 'aws login' from updating a profile with existing credentials

[ashovlin]

Issue #, if available: #9869

Description of changes:

This adds a prompt to aws login to warn users when running it for a profile that already has a different style of credentials:

$ aws login

Warning: Profile 'default' is already configured with Access Key credentials. If you continue to log in, the CLI and other tools may continue to use the existing credentials instead.

You may run 'aws login --profile new-profile-name' to create a new profile, or else you may manually remove the existing credentials from 'default'.

Do you want to continue adding login credentials to 'default'? (y/n):

This now prevents the user from proceeding if the profile already has existing credentials.

$ aws login

Error: Profile 'default' is already configured with Access Key credentials.

You may run 'aws login --profile new-profile-name' to create a new profile, or you must first manually remove the existing credentials from 'default'.

This should help avoid the confusing case where you run aws login but subsequent commands are still using different credentials.

I did not add support for clearing the other style of credentials yet. For access keys there wouldn't be an easy way to undo or retrieve them if we just delete them from the credentials file, so I'd want to be more careful here. If we get feedback, we could expand in the future.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[hssyoo]

Had a non-blocking comment but otherwise 🏆

[hssyoo]

non-blocking: It's a little weird that the command returns 0 and an error message is directed to stdout in this case.