Improve error messages when parsing headers.

Tested:
  Looked at output of failing tests.
PiperOrigin-RevId: 306031407

test/packetimpact/testbench/connections.go

@@ -213,7 +213,7 @@ func (conn *TCPIPv4) RecvFrame(timeout time.Duration) Layers {
 		}
 		layers, err := ParseEther(b)
 		if err != nil {
-			conn.t.Logf("can't parse frame: %s", err)
+			conn.t.Logf("debug: can't parse frame, ignoring: %s", err)
 			continue // Ignore packets that can't be parsed.
 		}
 		if !conn.incoming.match(layers) {

test/packetimpact/testbench/layers.go

@@ -153,7 +153,7 @@ func (l *Ether) toBytes() ([]byte, error) {
 			fields.Type = header.IPv4ProtocolNumber
 		default:
 			// TODO(b/150301488): Support more protocols, like IPv6.
-			return nil, fmt.Errorf("can't deduce the ethernet header's next protocol: %d", n)
+			return nil, fmt.Errorf("ethernet header's next layer is unrecognized: %#v", n)
 		}
 	}
 	h.Encode(fields)
@@ -191,7 +191,7 @@ func ParseEther(b []byte) (Layers, error) {
 		return append(layers, moreLayers...), nil
 	default:
 		// TODO(b/150301488): Support more protocols, like IPv6.
-		return nil, fmt.Errorf("can't deduce the ethernet header's next protocol: %#v", b)
+		return nil, fmt.Errorf("ethernet header's type field is unrecognized: %#04x", h.Type())
 	}
 }
 
@@ -274,7 +274,7 @@ func (l *IPv4) toBytes() ([]byte, error) {
 			fields.Protocol = uint8(header.UDPProtocolNumber)
 		default:
 			// TODO(b/150301488): Support more protocols as needed.
-			return nil, fmt.Errorf("can't deduce the ip header's next protocol: %#v", n)
+			return nil, fmt.Errorf("ipv4 header's next layer is unrecognized: %#v", n)
 		}
 	}
 	if l.SrcAddr != nil {
@@ -344,7 +344,7 @@ func ParseIPv4(b []byte) (Layers, error) {
 		}
 		return append(layers, moreLayers...), nil
 	}
-	return nil, fmt.Errorf("can't deduce the ethernet header's next protocol: %d", h.Protocol())
+	return nil, fmt.Errorf("ipv4 header's protocol field is unrecognized: %#02x", h.Protocol())
 }
 
 func (l *IPv4) match(other Layer) bool {