Error messages referring to inferred capabilities are not helpful

In the following example the generated error message refers to an implicitly inferred capability. It is never bound and impossible to understand:

```
effect Yield(): Unit

def foo() = {
var x in global = 42;
x = x + 1
def bar(): Unit / {} = do Yield();
println(x)
fun() { bar() }
}

def main() = try {
foo()
} with Yield { resume(()) }
```


The inferred type of `foo` also mentions the capability (which is equally hard to understand)


We need a proper way to refer to capabilities that are introduced for effects.