[net] Don't return an optional from TransportDeserializer::GetMessage()

Also, access mapRecvBytesPerMsgCmd with `at()` not `find()`. This
throws an error if COMMAND_OTHER doesn't exist, which should never
happen. `find()` instead just accessed the last element, which could make
debugging more difficult.

Resolves review comments from PR19107:

- https://github.com/bitcoin/bitcoin/pull/19107#discussion_r478718436
- https://github.com/bitcoin/bitcoin/pull/19107#discussion_r478714497

src/net.h

@@ -309,7 +309,7 @@ public:
     /** read and deserialize data, advances msg_bytes data pointer */
     virtual int Read(Span<const uint8_t>& msg_bytes) = 0;
     // decomposes a message from the context
-    virtual std::optional<CNetMessage> GetMessage(std::chrono::microseconds time, uint32_t& out_err) = 0;
+    virtual CNetMessage GetMessage(std::chrono::microseconds time, bool& reject_message) = 0;
     virtual ~TransportDeserializer() {}
 };
 
@@ -373,7 +373,7 @@ public:
         }
         return ret;
     }
-    std::optional<CNetMessage> GetMessage(std::chrono::microseconds time, uint32_t& out_err_raw_size) override;
+    CNetMessage GetMessage(std::chrono::microseconds time, bool& reject_message) override;
 };
 
 /** The TransportSerializer prepares messages for the network transport