Robocopy error code not included when failing during test

[magnustomte]

The following snippet in Test-TargetResource:

    else
    {
        throw "robocopy returned with errors! Exit code: $result! More info here:https://support.microsoft.com/en-us/kb/954404"
    }

Should be:

    else
    {
        throw "robocopy returned with errors! Exit code: $LASTEXITCODE! More info here:https://support.microsoft.com/en-us/kb/954404"
    }

as it is in the Set-TargetResource.

[Arturas-K]

@magnustomte good spot thanks.
Adding this fix to #23