Log protocol error causes at debug (#371)

Currently, there are many cases where `h2` will fail a connection or
stream with a PROTOCOL_ERROR, without recording why the protocol error
occurred. Since protocol errors may result from a bug in `h2` or from a
misbehaving peer, it is important to be able to debug the cause of
protocol errors.

This branch adds a log line to almost all cases where a protocol error
occurs. I've tried to make the new log lines consistent with the
existing logging, and in some cases, changed existing log lines to make
them internally consistent with other log lines in that module. All
receive-side errors that would send a reset are now logged at the debug
level, using a formatting based on the format used in `framed_read`.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/proto/streams/streams.rs

@@ -205,6 +205,7 @@ where
             } else {
                 if !frame.is_end_stream() {
                     // TODO: Is this the right error
+                    proto_err!(conn: "recv_headers: trailers frame was not EOS; stream={:?}", stream.id);
                     return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
                 }
 
@@ -231,7 +232,7 @@ where
                     return Ok(());
                 }
 
-                trace!("recv_data; stream not found: {:?}", id);
+                proto_err!(conn: "recv_data: stream not found; id={:?}", id);
                 return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
             },
         };
@@ -261,6 +262,7 @@ where
         let id = frame.stream_id();
 
         if id.is_zero() {
+            proto_err!(conn: "recv_reset: invalid stream ID 0");
             return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
         }
 
@@ -343,6 +345,10 @@ where
             // the value they send in the last stream identifier, since the
             // peers might already have retried unprocessed requests on another
             // connection.")
+            proto_err!(conn:
+                "recv_go_away: last_stream_id ({:?}) > max_stream_id ({:?})",
+                last_stream_id, actions.recv.max_stream_id(),
+            );
             return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
         }
 
@@ -427,7 +433,10 @@ where
                 stream.state.ensure_recv_open()?;
                 stream.key()
             }
-            None => return Err(RecvError::Connection(Reason::PROTOCOL_ERROR)),
+            None => {
+                proto_err!(conn: "recv_push_promise: initiating stream is in an invalid state");
+                return Err(RecvError::Connection(Reason::PROTOCOL_ERROR))
+            },
         };
 
         // TODO: Streams in the reserved states do not count towards the concurrency