Test throws

Coverage analysis suggests that it would be better to have tests for errors.

https://app.codecov.io/github/ohno/Antique.jl/commit/dea7528f66fc76aaf051e125ee37c13cc5039f42/tree/src?dropdown=coverage

`@test_throws` is available for test `throw`.

```julia
using Test
using Antique
HA = HydrogenAtom()
@test_throws DomainError V(HA, -1.0)
@test_throws DomainError E(HA, n=0)
```