Cosmetic changes in `CombinedErrorGenerator.kt`. (#806)

import software.amazon.smithy.rust.codegen.util.toSnakeCase

/**

 * For a given Operation ([this]), return the symbol referring to the unified error? This can be used

 * if you, eg. want to return a unfied error from a function:

 * For a given Operation ([this]), return the symbol referring to the unified error. This can be used

 * if you, e.g. want to return a unified error from a function:

 *

 * ```kotlin

 * rustWriter.rustBlock("fn get_error() -> #T", operation.errorSymbol(symbolProvider)) {

            errors.forEach { error ->

                val errorSymbol = symbolProvider.toSymbol(error)

                val fnName = errorSymbol.name.toSnakeCase()

                writer.rust("/// Returns true if the error kind is `${symbol.name}Kind::${errorSymbol.name}`.")

                writer.rust("/// Returns `true` if the error kind is `${symbol.name}Kind::${errorSymbol.name}`.")

                writer.rustBlock("pub fn is_$fnName(&self) -> bool") {

                    rust("matches!(&self.kind, ${symbol.name}Kind::${errorSymbol.name}(_))")

                }

    /**

     * Generates code to delegate behavior to the variants, for example:

     *

     * ```rust

     *  match self {

     *  match &self.kind {

     *      GreetingWithErrorsError::InvalidGreeting(_inner) => inner.fmt(f),

     *      GreetingWithErrorsError::ComplexError(_inner) => inner.fmt(f),

     *      GreetingWithErrorsError::FooError(_inner) => inner.fmt(f),