Add an error type

**Is your feature request related to a problem? Please describe.**
Need a simplified syntax for declaring an error type with an embedded struct and an embedded error type.

**Describe the solution you'd like**
Be able to write something like:
```daybreak
type IntErr error is int end
```
Should generate a struct equivalent to
```daybreak
type IntErr struct is
val: int
err: error ? Or maybe int for now
end
```

**Describe alternatives you've considered**
Implementing tuples for argument unpacking. I find that to be confusing and an unnecessary language complication.

**Additional context**
N/A