[libcalamares] When logging extra lines to warning or error, suppress label

Avoid logs like

23:29:57 [2]: void Config::setConfigurationMap(const QVariantMap&)
    WARNING: Configuration for *initialSwapChoice* is not one of the *userSwapChoices*
WARNING:    .. Choice "small" added.

where the label is duplicated.

src/libcalamares/utils/Logger.h

@@ -71,8 +71,11 @@ private:
 inline CDebug&
 operator<<( CDebug&& s, const FuncSuppressor& f )
 {
+    if ( s.m_funcinfo )
+    {
         s.m_funcinfo = nullptr;
-    s << f.m_s;
+        s.m_msg = QString( f.m_s );
+    }
     return s;
 }