Dick Costolo’s credibility is on the line. Twitter Inc.’s chief executive officer failed to foresee a slowdown that forced the social-media company to miss analysts’ first-quarter revenue estimates and cut its 2015 sales forecast, sending the stock tumbling. While this isn’t the first time Twitter has fallen short on promised results, investors had been told that new features and services, as well as a management overhaul, were starting to pay off.