Fix error struct with default impl (#3190)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

When an error member has a default value, it will generate errors
(specifically is_none not found on type String) with smithy-rs client
unless you manually specify @required

More Here: #3182

## Description
<!--- Describe your changes in detail -->

Adds recomendation change to address error structures with a default
implementation like so:

```kotlin
if (errorMessageMember != null) {
    val symbol = symbolProvider.toSymbol(errorMessageMember)
    if (symbol.isOptional()) {
        rust(
            """
            if tmp.message.is_none() {
                tmp.message = _error_message;
            }
            """,
        )
    }
}
..... 
```


```smithy
@error("client")
structure Error {
    @required
    requestId: String

    @required
    message: String

    code: String = "400"

    context: String
}
```

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

Added the above and ran `./gradlew codegen-client-test:build` with a
build successful

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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choice._

---------

Co-authored-by: John DiSanti <john@vinylsquid.com>