Disallow ignoring a constrained shape in an event stream's closure (#2113)

The server SDK will otherwise produce Rust code that cannot be compiled,
so it doesn't make sense to prompt users to disregard the warning and
opt into `ignoreUnsupportedConstraints`. To continue synthesizing the
model, the user must remove any constrained traits from the event
stream's closure.

This commit also tweaks formatting of errors yielded by
`ValidateUnsupportedConstraints`. Prior to this commit, they were a bit
off due to carelessness when interpolation occurs. For example:

```
[SEVERE] Operation com.amazonaws.constraints#ConstrainedShapesOperation takes in input that is constrained(https://awslabs.github.io/smithy/2.0/spec/constraint-traits.html), and as such can fail with a validationexception. You must model this behavior in the operation shape in your model file.
```