Improve credential errors for github-likes

[mih]

Previously, when a credential was auto-discovered or entered -- but then access failed -- identical errors meesages were produced:

Credential did not yield successful authorization

without any advice on what to do.

With this change, the error message is different, depending on whether a credential was automatically determined from a query, or whether it was entered manually right before the access attempt. Now it would say:

Auto-selected existing credential:

Auto-selected did not yield successful authorization.
Please select a different credential via the credential option of
this command, or remove/edit the credential with the DataLad command
credentials.

Entered credential:

Entered did not yield successful authorization.
Please try again with a valid credential.

In both cases the underlying reason for the unsuccessful attempt (generally a 401 response code) continues to be reported.

Now a user can be better aware how credential selection can be performed, and that there is a credential management command.

The idea of an interactive enter-try-fail-reenter loop to guide a user has been rejected, because of the implied complexity from possible mitigation strategies. Importantly, the fact that an existing credential does not work for a particular operation is insufficient for concluding that a credential is completely invalid (hence could be replaced). It may simply be insufficient for the target operation, but remains useful or needed for others.