Add strategy for handling download error

[laistomazz]

Recently I spent a couple of hours going through both my AWS and the plugin's configuration to try and figure out why I was having errors when downloading my credentials.

In the end it was the simple fact that my Downloads directory already contained a folder called credentials. The conflictAction overwrite does not handle such situation.

So I have the following proposal: when an error occurs, show a dialog window and give the user the option to retry the download, but this time with prompt as a conflictAction strategy. In my case, that would have showed me right away the reason why I was having the error.