Add debug logs for http2 stream errors (#4515)

Motivation:

We sometimes encounter situations where  HTTP/2 header frames are lost due to an exception.
Although a RST frame is sent in this case, remotes are still able to send data for the offending stream, which ends up in error logs which armeria doesn't understand.

e.g.
```
io.netty.handler.codec.http2.Http2Exception: Stream 1147 does not exist
```

Although normally it might be noisy to be alerted of such failures, it might be useful to view such logs when debugging.

Modifications:

- Added logging for HTTP/2 `StreamError`
- Added logging for connection errors just in case as well.

Result:

- Easier to debug http2 failure logs.

core/src/main/java/com/linecorp/armeria/internal/common/AbstractHttp2ConnectionHandler.java

@@ -45,6 +45,7 @@
 import io.netty.handler.codec.http2.Http2ConnectionEncoder;
 import io.netty.handler.codec.http2.Http2ConnectionHandler;
 import io.netty.handler.codec.http2.Http2Exception;
+import io.netty.handler.codec.http2.Http2Exception.StreamException;
 import io.netty.handler.codec.http2.Http2Settings;
 import io.netty.handler.codec.http2.Http2Stream.State;
 import io.netty.handler.codec.http2.Http2StreamVisitor;
@@ -111,13 +112,20 @@ protected final void onConnectionError(ChannelHandlerContext ctx, boolean outbou
 
         handlingConnectionError = true;
         if (Exceptions.isExpected(cause) || isGoAwaySentException(cause, connection())) {
-            // Ignore silently.
+            logger.trace("{} HTTP/2 connection error:", ctx.channel(), cause);
         } else {
             logger.warn("{} HTTP/2 connection error:", ctx.channel(), cause);
         }
         super.onConnectionError(ctx, outbound, cause, filterHttp2Exception(cause, http2Ex));
     }
 
+    @Override
+    protected void onStreamError(ChannelHandlerContext ctx, boolean outbound, Throwable cause,
+                                 StreamException http2Ex) {
+        logger.debug("{} HTTP/2 stream error:", ctx.channel(), cause);
+        super.onStreamError(ctx, outbound, cause, http2Ex);
+    }
+
     private static Http2Exception filterHttp2Exception(Throwable cause, @Nullable Http2Exception http2Ex) {
         if (http2Ex != null) {
             return http2Ex;