Change the default value for ArcanistExternalLinter::shouldExpectCommandErrors

Summary: It is more common for linters to exit with a non-zero status than it is for linters to return with a zero exit status, Really this function serves very little purposes, it simply determines whether or not to throw an exception if a non-zero status is returned by the external linter.

Test Plan: arc unit

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11322