Fix error handling for custom executables.

Prevent dialog showing "No executable" when canceling the
prompt for the executable when pressing 'Run' / 'Debug'
on a library project. Introduce new
LocalApplicationRunControl::ensureConfigured() to ensure
the configuration is complete and detect cancel via
empty/non-null strings.

Change-Id: I9bd4a296e7c995d26d6ad265519e7ebd3f98d6fe
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/projectexplorer/runconfiguration.cpp

@@ -322,6 +322,16 @@ bool RunConfiguration::isConfigured() const
     return true;
 }
 
+bool RunConfiguration::ensureConfigured(QString *errorMessage)
+{
+    if (isConfigured())
+        return true;
+    if (errorMessage)
+        *errorMessage = tr("Unknown error.");
+    return false;
+}
+
+
 /*!
     \fn virtual QWidget *ProjectExplorer::RunConfiguration::createConfigurationWidget()