objexcept: No more magic messages in exceptions, only exception arguments.

One of the reason for separate "message" (besides still unfulfilled desire to
optimize memory usage) was apparent special handling of exception with
messages by CPython. Well, the message is still just an exception argument,
it just printed specially. Implement that with PRINT_EXC printing format.
objfloat: Missing default: caused incorrect results for unimplemented ops.
objfloat: Make sure that floats always have dot (for C "double" type case).

This matches CPython behavior and hopefully can be treated as general
Python semantics.
compile: Don't try to constant-fold division by zero.

The way it is, just crashes app. And optimizing to
"raise ZeroDivisionError" is probably too much.
objfloat: Quick&dirty implementation of float floor division.

TODO: Likely doesn't match Python semantics for negative numbers.
py: Properly implement divide-by-zero handling.

"1/0" is sacred idiom, the shortest way to break program execution
(sys.exit() is too long).