Added new test for targetFilePath

Added a new test for when a targetFilePath to a location that does not
exist is provided.  This should result in a failure, and a call to
Write-ChocolateyFailure.
Added try/catch

Added a try/catch block around the main function which creates the
shortcut.
Added Debug Statements

Added some desbug statements to indicate where in the helper method has
been reached.
Added test to verify shortcut created

Created a new test to verify that a desktop shortcut is correctly
created.  Added some clean up steps to remove the files that are created
as part of the test.