Soft deprecation of LoadError in runtime
Remove use of LoadError from the runtime now that backtraces are more
reliable. It's left in boot.jl for now for 1.x compatibility.
Compatibility:
It's difficult to deprecate this in a fully backward compatible way,
because packages commonly test macro expansion using constructs like
@ test_throws LoadError macroexpand(:(@ some_pkg_macro))
to make this into as minor a change as possible, grandfather special
rules for LoadError into stdlib/Test.
