Proper handling of credentials not found for docker-credential-desktop

[fopina]

Can't pull any public image as this store returns a slightly different message than the one hardcoded, raising the wrong exception (StoreError instead of CredentialsNotFound)

echo 'https://index.docker.io/v1/' | docker-credential-desktop get
No stored credential for https://index.docker.io/v1/

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" git@github.com:fopina/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[fopina]

appveyor tests failing seem to be related to their default setup having changed..

[fopina]

Should I close this?