Make C struct match the binary API more closely

Also make the internal conversion function return errors properly

include/zaprus.h

@@ -20,13 +20,12 @@ struct SaprusMessage* zaprus_connect(const char* payload, size_t len);
 
 struct SaprusMessage {
   uint16_t packet_type;
+  uint16_t payload_len;
   union {
     struct {
       struct {
 	char dest[4];
       };
-      size_t payload_len;
-      char *payload;
     } relay;
     struct {
       struct {
@@ -37,10 +36,10 @@ struct SaprusMessage {
 	char _reserved;
 	char options;
       };
-      size_t payload_len;
-      char *payload;
+      
     } connection;
-  };
+  } headers;
+  char *payload;
 };
 
 // ptr should be freed by the caller.