chore: Update error message when fail to send log (#790)

## Problem
When logs fail to be sent to Datadog, the extension prints an error:
> No more retries as the access was rejected

which I think can be more clear.

<img width="709" height="305" alt="Pasted Graphic"
src="https://github.com/user-attachments/assets/6eae0497-8b24-48d0-a192-5215c6e1560f"
/>


## This PR
Change the error message to:
> Failed to send logs to datadog because access was rejected. Is the API
key invalid?

Author: lym953

bottlecap/src/logs/flusher.rs

@@ -128,7 +128,10 @@ impl Flusher {
                     let status = resp.status();
                     _ = resp.text().await;
                     if status == StatusCode::FORBIDDEN {
-                        error!("No more retries as the access was rejected");
+                        // Access denied. Stop retrying.
+                        error!(
+                            "Failed to send logs to Datadog: Access denied. Please verify that your API key is valid."
+                        );
                         return Ok(());
                     }
                     if status == 202 {