ProjectExplorer: Replace the fromMap error return value

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

The driving reason is to get the same fromMap signature as in the
AspectContainer base, however, the whole mechanism seems rather useless:
There are only a few places that actually ever could report errors,
in those places "moving on" looks ok, too, and these few places
are not conceptually different than a lot others.

Removal of this (new) mechanism is left for later.

Change-Id: Ibe7a0456bc3b84a84e942231f14446e783372d76
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/mesonprojectmanager/mesonbuildconfiguration.h

@@ -31,7 +31,8 @@ signals:
 
 private:
     void toMap(QVariantMap &map) const override;
-    bool fromMap(const QVariantMap &map) override;
+    void fromMap(const QVariantMap &map) override;
+
     MesonBuildType m_buildType;
     ProjectExplorer::NamedWidget *createConfigWidget() final;
     MesonBuildSystem *m_buildSystem = nullptr;