Improve auth error message

[thomas11]

• Fix missing values in auth error messages coming from upstream
• Update link to Pulumi docs
• Shorten a bit

Before (note second line "for ..." and third line "see ..."):

    error: unable to validate AWS credentials.
    Details: no valid credential sources for  found.

    Please see
    for more information about providing credentials.

    Error: <actual error>


    Make sure you have:

     	 • Set your AWS region, e.g. `pulumi config set aws:region us-west-2`
     	 • Configured your AWS credentials as per https://pulumi.io/install/aws.html
     	 You can also set these via cli using `aws configure`.

After:

    error: unable to validate AWS credentials.
    Details: no valid credential sources for Pulumi AWS Classic found.

    Please see https://www.pulumi.com/registry/packages/aws/installation-configuration/
    for more information about providing credentials.

    Error: <actual error>

    Make sure you have set your AWS region, e.g. `pulumi config set aws:region us-west-2`.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[rquitales]

LGTM

[kpitzen]

LGTM!

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.