Type errors should mention return type if it's provided

[oskgo]

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.

[strub]

Yes, the error message comes from a time where the context was not used. It is now not informative.