Improve user experience for setting PATH in Windows

[leezen]

A few thoughts:

• Currently, if updating the PATH in the registry fails, there's no warning/error message:
  

  get.pulumi.com/dist/install.ps1

  Line 61 in e0415f1

  } catch {

• It's not clear that the user may have to sign out and back in for the PATH setting to stick. So, if someone installs pulumi and opens a new cmd.exe or PowerShell window, pulumi will not be on the path.

[leezen]

The second bullet is addressed by #80