update(engine): clarify override error messages

Signed-off-by: Luca Guerra <luca@guerra.sh>

userspace/engine/rule_loader_reader.cpp

@@ -441,10 +441,11 @@ static void read_item(
 		std::set<std::string> override_append, override_replace;
 		std::set<std::string> overridable {"items"};
 		decode_overrides(item, overridable, overridable, override_append, override_replace, ctx);
+		bool has_overrides = !override_append.empty() || !override_replace.empty();
 
-		if(append == true && !override_replace.empty())
+		if(append == true && has_overrides)
 		{
-			THROW(true, "Cannot specify a replace override when 'append: true' is set", ctx);
+			THROW(true, "Keys 'override' and 'append: true' cannot be used together.", ctx);
 		}
 
 		// Since a list only has items, if we have chosen to append them we can append the entire object
@@ -482,10 +483,11 @@ static void read_item(
 		std::set<std::string> override_append, override_replace;
 		std::set<std::string> overridable {"condition"};
 		decode_overrides(item, overridable, overridable, override_append, override_replace, ctx);
+		bool has_overrides = !override_append.empty() || !override_replace.empty();
 
-		if(append == true && !override_replace.empty())
+		if(append == true && has_overrides)
 		{
-			THROW(true, "Cannot specify a replace override when 'append: true' is set", ctx);
+			THROW(true, "Keys 'override' and 'append: true' cannot be used together.", ctx);
 		}
 
 		// Since a macro only has a condition, if we have chosen to append to it we can append the entire object
@@ -523,7 +525,8 @@ static void read_item(
 		bool has_overrides_replace = !override_replace.empty();
 		bool has_overrides = has_overrides_append || has_overrides_replace;
 
-		THROW((has_append_flag && has_overrides), "Keys 'override' and 'append' cannot be used together.", ctx);
+		THROW((has_append_flag && has_overrides),
+			"Keys 'override' and 'append: true' cannot be used together. Add an append entry (e.g. 'condition: append') under override instead.", ctx);
 
 		if(has_overrides)
 		{