Nikkei tumbles on Greek 'No' vote, yen's rise; Toshiba dives

* Toshiba dives 5 pct on renewed fears on accounting errors * ECB's safety net likely limit contagion - analyst * Kuroda's comment on stimulus triggers hopes for ETF buying - analyst By Ayai Tomisawa TOKYO, July 6 (Reuters) - Japan's Nikkei share average fell on Monday, with financial stocks and exporters leading the declines, after Greeks overwhelmingly rejected austerity measures demanded in return for bailout money, throwing the euro zone into turmoil. The Nikkei share a