Invalid missingUncooked entries produce confusing error

[pledbrook]

If the build can't find a file that's listed in the missingUncooked array, you get an error like this:

Copying requested uncooked packages...
Including MissingUncooked
ForEach-Object : The property 'FullName' cannot be found on this object. Verify that the property exists.
At E:\Games\Mods\lwotc\.scripts\X2ModBuildCommon\build_common.ps1:148 char:11
+         $this | ForEach-Object $stepCallback
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [ForEach-Object], PropertyNotFoundException
    + FullyQualifiedErrorId : PropertyNotFoundStrict,Microsoft.PowerShell.Commands.ForEachObjectCommand

It would be better to have a clearer message that the file cannot be found.