Reject source module names which don't correspond to an existing module

[serin-delaunay]

Currently it is easy for a user to write a module group like this:

{
    "defaultoutputmodule": "m",
    "modules": {
        "m": {
            "type": "Abs",
            "SourceModule": "NONEXISTENT"
        }
    }
}

This will cause Wangscape to crash (without printing an error message, on Windows), and is not detected by Wangcheck.

Such module groups should fail Wangcheck.