Reduce the usage of projectLLVM_bv

[RyanGlScott]

The Crucible-LLVM function projectLLVM_bv takes a pointer, asserts that it is a bitvector, and returns the bitvector value. The problem is that when this assertion fails, it does so with a generic and unhelpful error message. stubs should avoid this function, and instead provide more detailed messages that match the level of abstraction of the program under test (e.g., "attempted to divide by a pointer").

See GaloisInc/crucible#1220 for how this was done in Crucible-LLVM.