Shorten strings in PhutilReadableSerializer::printShort()

Summary: We currently don't shorten strings, which can cause functions like "insert_into_database($giant_block_of_data)" to produce enormous and almost-certainly-useless logs. Shorten overly-long strings.

Test Plan: Threw an exception from f(str_repeat('y', 1024 * 1024)), verified the resulting trace wasn't a gigantic mess.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2193

Differential Revision: https://secure.phabricator.com/D4209