PowerShell - reliably detect failure from external scripts

exec.ps1

<#
.Synopsis
Detects failure from an external script has failed based on both $? and $LASTEXITCODE checking
Tested with a .net console app in 4 modes:
- no stderr, exit 0 (should be the only passing case)
- stderr, exit 0 ($? should indicate failure)
- no stderr, exit 1 ($LASTEXITCODE should indicate failure)
- stderr, exit 1 (both $? and $LASTEXITCODE should indicate failure)
See http://stackoverflow.com/questions/10666101/lastexitcode-0-but-false-in-powershell-redirecting-stderr-to-stdout-gives-n for background
#>
function exec($exe){
    $eap = $ErrorActionPreference;
    $global:LASTEXITCODE = 0
    try{
        # Temporarily change EAP
        $ErrorActionPreference = 'continue';

        # Redirection of the error to std out is neccesary
        # to force PS console (not ISE) to monitor the error stream
        # and thus trip $? if errors occur
        & $exe $args 2>&1 | % { "$_" }

        # Store $? in case any of the other statements change it
        $errorsRaised = !$?
    }finally{
        $ErrorActionPreference = $eap;
    }
    if($global:LASTEXITCODE -ne 0){
        write-warning ('Failed with exit code ' + $global:LASTEXITCODE)
    }elseif($errorsRaised){
        write-warning 'Failed (errors written to std out or non-zero exit code)'
    }
}