Improved payload Deser error messages (#905)

* made error messages more informative
* added a suggestion to log at TRACE level to README

Author: rimutaka

README.md

@@ -306,7 +306,9 @@ async fn main() -> Result<(), Error> {
 }
 ```
 
-The subscriber uses `RUST_LOG` as the environment variable to determine the log level for your function. It also uses [Lambda's advance logging controls](https://aws.amazon.com/blogs/compute/introducing-advanced-logging-controls-for-aws-lambda-functions/) if they're configured for your function. By default, the log level to emit events is `INFO`.
+The subscriber uses `RUST_LOG` environment variable to determine the log level for your function. It also uses [Lambda's advanced logging controls](https://aws.amazon.com/blogs/compute/introducing-advanced-logging-controls-for-aws-lambda-functions/), if configured.
+
+By default, the log level to emit events is `INFO`. Log at `TRACE` level for more detail, including a dump of the raw payload.
 
 ## AWS event objects
 

lambda-runtime/src/layers/api_response.rs

@@ -98,7 +98,7 @@ where
             Ok(())
         };
         if let Err(err) = trace_fn() {
-            error!(error = ?err, "failed to parse raw JSON event received from Lambda");
+            error!(error = ?err, "Failed to parse raw JSON event received from Lambda. The handler will not be called. Log at TRACE level to see the payload.");
             return RuntimeApiResponseFuture::Ready(Some(Err(err)));
         };
 
@@ -124,7 +124,7 @@ fn build_event_error_request<'a, T>(request_id: &'a str, err: T) -> Result<http:
 where
     T: Into<Diagnostic<'a>> + Debug,
 {
-    error!(error = ?err, "building error response for Lambda Runtime API");
+    error!(error = ?err, "Request payload deserialization into LambdaEvent<T> failed. The handler will not be called. Log at TRACE level to see the payload.");
     EventErrorRequest::new(request_id, err).into_req()
 }