Improve error handling

[jbourassa]

• Refactor: remove the need to hold host exception on the store, downcast the exception instead.
• Add missing trap codes for Wasm traps.
• Improve experience of param and result
  • Introduce a new ResultError for when a host call's result can't be converted. The exception message contains Proc#inspect -- it's verbose but I suspect this will help a lot when debugging.
  • Change Wasmtime::Error to ArgumentError when calling a Wasm function with incorrect arity.
  • Specify which param or result failed to convert by adding its index to the error message.