webui extension publishing improvements

[amvanbaren]

💡 #1 a nitpick I though of is that when an error like this one happens (an error with the package, not our service), we could reset the input so that the file does not display anymore. Something for later maybe :)

💡 #2 We could also add a heads up when this flow has created a namespace, or, maybe even better, before it would have created the namespace display a prompt asking the user whether to really create namespace x.

💡 #3: we could be more helpful when publishing an extension where the namespace is already claimed. We could add the user name of the user who owns it (so that the user can contact the owner) and also explain that the owner can add the user to the namespace and how.

source: https://github.com/eclipse/openvsx/pull/616#pullrequestreview-1188754581