Opening up a pie shop is hard enough. Throw in an abrupt closure from your crowdfunder, and that might make you crazy. That’s exactly what happened to Elias Lehnert, a fourth-generation piemaker who owns Legacy Pie Co., when Mainvest, a Massachusetts-based equity crowdfunding site, announced it was closing. At the end of May, Mainvest sent an email to Lehnert and all his investors to announce its shuttering, he said.