fix: gracefully handle non-record on LHS of field expression

Previously, we would assert-fail here because this was considered a violation of
our assumptions. However, post commit d936b4e4
(or more generally, any of the ambiguous operator work), type() can be invoked
*during* type resolution, causing it to be executed on a potentially badly typed
model. We now need to throw exceptions instead of assert-failing in such
situations.

Github: fixes #190 "assertion failure with apartment.m variant"