Move prompt gathering out of GetSessionDetails

[daviwil]

When I introduced the concept of gathering session details after each command that gets run, I rolled prompt gathering into that to avoid running yet another command each time a prompt is needed. It seems that this causes more problems than necessary because a prompt function that throws errors does not get handled correctly anymore.

I'll move prompt gathering back out into the ConsoleService so that we can check for errors and return a default prompt appropriately.

[daviwil]

Related to this issue comment: https://github.com/PowerShell/vscode-powershell/issues/652#issuecomment-292048745