Improve PHP7 handling of include-time parser-like fatals like "class must implement all abstract methods"

Summary:
Ref T12101. Although PHP7 raises parse errors as throwables, it still just instantly fatals for some types of parser-like error.

We currently silence these, so they can cause the process to appear to exit abruptly. Instead, unsilence them.

Test Plan:

• Don't implement all abstract methods.
• Run arc unit.
• Before: silent fatal.
• After: useful error.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12101

Differential Revision: https://secure.phabricator.com/D19001