Lift function to "error" statement

Currently we are doing

```go
it.Then(t).Should(
		it.Fail(
			func() error {
				return json.Unmarshal([]byte(geometryUnknown), geo)
			},
		),
	)
```

it would be nice to

```go
it.Then(t).Should(
		it.Fail(json.Unmarshal([]byte(geometryUnknown), geo)),
	)
```
