Block access to the BuildDirManager while one of its errors is
processed. This prevents more errors being raised as part of
error handling, which can trigger a loop.

Task-number: QTCREATORBUG-17869
Change-Id: Ic6f8d9a3c3b4e63f27260c40f27ab09d20b62b3e
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>