This change fixes two problems in our powershell scripts

1. Removes all uses of Invoke-Expression.  This commandlet doesn't do error checking on windows
commands and hence can hide failures.  Relpaced with Exec-Expression which properly checks
both the windows command error and the powershell error
1. Reset $LastExitCode inside Exec before executing the script block.  This avoids cases where
previous windows command failures could carry over to a script block that didn't execute any
windows commands.

Powershell guidelines changed appropriately