Reify message traps as errors to match platform

[crusso]

Our interpreters abort on traps instead of propagating them as failure messages.

[nix-shell:~/motoko/test/run]$ rlwrap ../../src/moc
Motoko 0.1 interpreter
> import Prim "mo:prim";
let Prim : module {...}
> let a = async throw Prim.error("foo");
let a : async<$top-level> None = async Error (#canister_reject, "foo")
> let b = async 1/0;
stdin:3.15-3.18: execution error, arithmetic overflow
let b : async<$top-level> Nat = async _
> await b;
> b;
async[1] _ : async<$top-level> Nat
> 

Related to #506.

The fix is probably to maintain an extra trap continuation in the interpreter environment, invoked from the current aborting trap function and PrimE