Check for trailing slash on -WSAppPath parameter

[spedinfargo]

Super helpful tool! Ran into a gotcha that isn't worth fixing, but just throwing it here as an issue in case someone else runs into it.

If you have a trailing slash on the -WSAppPath parameter, you'll get a non-intuitive error from the MakeAppx call. Took me a few minutes to figure that out, so hoping this helps someone else out in the future!