gom: Fix parameter name in header to match implementation

Was called "URI" in the implementation, but "filename" in the header.
We actually expect a URI, so change the header.
gom: Show an error when migration func is broken

In addition to asserting later in the code, throw a warning
mentioning if the external and developer-provided migration function
is broken, and fails to send back a GError when it fails.

This should stop people from pointing at a gom bug when their code
is broken (as I did :)
gom: Fix copying of values

Before being copied to using g_value_copy(), the destination
GValue needs to be initialised to the correct type. 2 places in
GomFilter were not doing that.
tests: Add test for the find function