Improve error messages

#### Feature

Improve error messages.

#### Benefit

Descriptive error messages are useful when writing raw wasm code.

#### Implementation

N/A

#### Alternatives

Not much of an alternative, but one can use wabt. For comparison, wasmtime:

```
$ wasmtime stack-types.wat
Error: WebAssembly translation error

Caused by:
Invalid input WebAssembly code at offset 45: type mismatch: expected i32 but nothing on stack
```

wabt:

```
$ wat2wasm stack-types.wat
stack-types.wat:3:5: error: type mismatch in i32.eqz, expected [i32] but got []
i32.eqz
^^^^^^^
```