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/cmakeprojectmanager/cmakebuildconfiguration.h

@@ -90,11 +90,7 @@ signals:
     void signingFlagsChanged();
     void configureEnvironmentChanged();
 
-protected:
-    bool fromMap(const QVariantMap &map) override;
-
 private:
-    void toMap(QVariantMap &map) const override;
     BuildType buildType() const override;
 
     ProjectExplorer::NamedWidget *createConfigWidget() override;