bb.build: in _exec_task, catch errors from TaskStarted

bb.build: in _exec_task, catch errors from TaskStarted

We don't always want a traceback when an exception is raised by the
TaskStarted event handler. Silently return if we get a SystemExit or
HandledException, and print the error and return for FuncFailed.

This is done via a separate try/catch block, to avoid firing TaskFailed if all
the TaskStarted event handlers didn't complete, otherwise the bitbake UIs get
unhappy.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>