Make 'repo sync -jN' exit with an error code in the case of sync errors.

The bug that this is fixing is described here:

This fix allows the helper threads to signal the main thread that they
saw an error.  When the main thread sees the error, it will let all
existing threads finish, then exit with an error.

Change-Id: If3019bc6b0b3ab9304d49ed2eea53e9d57f3095a
1 file changed