Slightly improve error reporting in two cases.

[alecthomas]

Caveat emptor: I really have no idea if this is the right approach in either case :)

1. When wasmtime returns a non-Trap error, include that error's string repr in the unhandled case.
2. When a guest calls spawn with a params array that is not a multiple of 17, return an error.

[bkolobara]

Looks good. Thanks for the contribution @alecthomas!