While refactoring object.cpp I noticed that gjs_define_object_class()
doesn't do proper error checking; the JSAPI functions used within it can
throw, but it doesn't check that.

Fixing this also had a cascade effect on
gjs_object_define_static_methods() and gjs_define_param_class(), as well
as some code in fundamental.cpp and repo.cpp.

Similarly, find_vfunc_info() in object.cpp could also throw but the
return value was never checked.