Avoid using InterceptorError in Interceptor impl

This commit incorporates the following feedback:
#2577 (comment)
#2577 (comment)
#2577 (comment)
#2577 (comment)

Author: ysaito1001

codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/endpoint/generators/EndpointParamsInterceptorGenerator.kt

@@ -83,10 +83,10 @@ class EndpointParamsInterceptorGenerator(
             let input = context.input()?;
             let _input = input
                 .downcast_ref::<${operationInput.name}>()
-                .ok_or_else(|| #{InterceptorError}::invalid_input_access())?;
+                .ok_or_else(|| "failed to downcast to ${operationInput.name}")?;
             let params_builder = cfg
                 .get::<#{ParamsBuilder}>()
-                .ok_or(#{InterceptorError}::read_before_execution("missing endpoint params builder"))?
+                .ok_or_else(|| "missing endpoint params builder".to_owned())?
                 .clone();
             ${"" /* TODO(EndpointResolver): Call setters on `params_builder` to update its fields by using values from `_input` */}
             cfg.put(params_builder);
@@ -111,7 +111,7 @@ class EndpointParamsInterceptorGenerator(
             )
             withBlockTemplate(
                 "let endpoint_prefix = ",
-                ".map_err(#{InterceptorError}::read_before_execution)?;",
+                """.map_err(|err| format!("endpoint prefix could not be built: {err:?}"))?;""",
                 *codegenScope,
             ) {
                 endpointTraitBindings.render(
@@ -130,11 +130,11 @@ class EndpointParamsInterceptorGenerator(
             let _ = context;
             let params_builder = cfg
                 .get::<#{ParamsBuilder}>()
-                .ok_or(#{InterceptorError}::read_before_execution("missing endpoint params builder"))?
+                .ok_or_else(|| "missing endpoint params builder".to_owned())?
                 .clone();
             let params = params_builder
                 .build()
-                .map_err(#{InterceptorError}::read_before_execution)?;
+                .map_err(|err| format!("endpoint params could not be built: {err:?}"))?;
             cfg.put(
                 #{EndpointResolverParams}::new(params)
             );