Notation login returns unfriendly error

[toddysm]

Related to the following comment on the documentation #149 (comment) trying to sign into Docker Hub I get the following error:

~> notation login docker.io                                                                              
Error: failed to store credentials: error storing credentials - err: no credentials username, out: `no credentials username`

We need to fix the error message and provide consistent experience with other tools to prompt for the username and password instead.

[FeynmanZhou]

@toddysm I think this issue has been covered by notaryproject/notation#503. There is already a corresponding PR to address this login experience at notaryproject/notation#566. We plan to improve this experience in RC.4.

If this issue is a feature request to Notation instead of a doc change, it duplicates with notaryproject/notation#503. We should consider closing it or transferring it to the Notation repo.

[toddysm]

I am fine closing this one in this repo but I would like to make sure that notaryproject/notation#503 and notaryproject/notation#566 implement better error messages.

[yizha1]

@toddysm Is it OK to close this issue, since notaryproject/notation#503 and notaryproject/notation#503 were closed.

[zr-msft]

@toddysm @yizha1 closing this issue