Leaving the euro: An emergency exit for the currency union?

The euro does not provide its members with any option to leave. This protects the common currency against speculative attacks on the one hand. The euro crisis demonstrated on the other hand how difficult it is for Eurozone countries to constructively solve economic and fiscal policy conflicts among themselves. Why was the euro designed as a oneway
street? What would be the consequences of an exit option? And what alternatives are there to an Exit?