[expr parser] Communicate expression operator table bake errors through callback.

source/md.h

@@ -790,7 +790,6 @@ struct MD_ExprOprTable
 {
     // TODO(mal): @upgrade_potential Hash?
     MD_ExprOprList table[MD_ExprOprKind_COUNT];
-    MD_MessageList errors;
 };
 
 typedef struct MD_Expr MD_Expr;
@@ -833,6 +832,8 @@ struct MD_ExprParseCtx
     MD_MessageList errors;
 };
 
+typedef void (*MD_BakeOperatorErrorHandler)(MD_MessageKind kind, MD_String8 s);
+
 //~ String Generation Types
 
 typedef MD_u32 MD_GenerateFlags;