The solar industry should have had a good week, with interest rates continuing to fall and the residential solar industry looking like it's about to turn a corner. Instead, SunPower's (SPWR 1.12%) shares collapsed 27%, according to data provided by S&P Global Market Intelligence, because SunPower warned investors that it may not be a "going concern" if it can't find additional financing.The financing crunchThis week, SunPower announced it had breached covenants on debt that could be immediately callable, me ...