ConfigInstructions should throw an exception when a DependsOn points to a non-existent resource

[alx9r]

Consider the following:

$document = {
    Get-DscResource TestStub | Import-DscResource

    TestStub a @{
        Key = 'a'
        DependsOn = '[TestStub]b'
    }
}

ConfigInstructions Name $document

[TestStub]a depends on [TestStub]b which does not exist. This is a user error and should throw an informative exception accordingly.

A test for this behavior is already mentioned in ConfigDocumentType.Tests.ps1.