ENH: refine check logic for filament mix printing

jira: NONE

Signed-off-by: xun.zhang <xun.zhang@bambulab.com>
Change-Id: I199462cee6284e13e58f829b7972dd3124bfc385
(cherry picked from commit 22834b23586d2230e94e9dffd8b09146e5342246)

src/libslic3r/PrintBase.hpp

@@ -33,6 +33,7 @@ struct StringObjectException
     ObjectBase const *object = nullptr;
     std::string opt_key;
     StringExceptionType         type;   // warning type for tips
+    bool is_warning = false;
     std::vector<std::string>    params; // warning params for tips
 };