delete.term can lead to unnamed constructors

[ceedubs]

Describe and demonstrate the bug

delete.term can be used to delete a type's constructor, resulting in an unnamed hash appearing in the type and a result in the todo list.

I think that instead it should fail letting you know that the constructor has references and for bonus points suggesting that you use delete.type on the type instead.

Input:

```unison
type MyType = MyConstructor
```

```ucm
fresh/main> add
fresh/main> delete.term MyType.MyConstructor
fresh/main> view MyType
fresh/main> todo
```

Output:

``` unison
type MyType = MyConstructor
```

``` ucm

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      type MyType

```
``` ucm
fresh/main> add

  ⍟ I've added these definitions:
  
    type MyType

fresh/main> delete.term MyType.MyConstructor

  Done.

fresh/main> view MyType

  type MyType = #326g7u3sau#0

fresh/main> todo

  These types have some constructors with missing names.
  
    1. MyType
  
  You can use `view 1` and
  `alias.term <hash> <TypeName>.<ConstructorName>` to give names
  to each unnamed constructor.

```

Environment (please complete the following information):

• ucm --version 1e44418