Type errors should mention return type if it's provided

The following code errors out with ``unknown variable or constant: `x'``, which is clearly false. The issue is that there's none with the desired type.
```
op x: bool.

op y: int = x.
```
This can be especially confusing when it happens in functions. The following example complains about there being no `f` with boolean parameter:
```
op f: bool -> bool.

op y: int = f true.
```