ProjectExplorer: Replace the fromMap error return value for toolchains

... by some out-of-band mechanism, keeping previous functionality.

The driving reason is to get the same fromMap signature as in the
AspectContainer base.

Change-Id: Ife7fbf4d0e0f78d88729021eb5220ec4a8ddb2ec
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/projectexplorer/toolchainsettingsaccessor.cpp

@@ -320,11 +320,10 @@ public:
         return static_cast<const TTC *>(&other)->token == token;
     }
 
-    bool fromMap(const QVariantMap &data) final
+    void fromMap(const QVariantMap &data) final
     {
         ToolChain::fromMap(data);
         token = data.value(TestTokenKey).toByteArray();
-        return true;
     }
 
     void toMap(QVariantMap &data) const final