reference class compile errors on load_all prevent further load_all

[kcritz]

when using reference classes, any compile error on load_all creates an error on all subsequent load_all calls:

Error in (function (class_a, pkg_a, class_b, pkg_b) :
trying to get slot "subclasses" from an object of a basic class ("NULL") with no slots

Speaking with Hadley at the R masterclass, he said that the problem can happen because load_all isn't properly unloading the flawed class. This may be related to the fact that reset=FALSE confusingly fixes the problem.