Treat all PHP language-level errors as exceptions by default

Summary:
Ref T13499. Currently, we throw most language errors as exceptions, but this list isn't exhaustive and errors we don't specifically make more severe are allowed to slip through.

This is generally undesirable, particularly in the case of "Undefined index:" errors. See T13499 for a specific case where this caused behavior to be more difficult to understand and diagnose than it should have been.

Make this the default behavior instead, except for "E_USER" errors, which we never expect to arise from first-party code.

This may be slightly too aggressive, but future changes can selectively reduce the severity of some types of errors if problems arise.

Test Plan:

• Executed code which intentionally accessed an undefined index, got an exception.
• Poked around Phabricator and Arcanist without any further issues cropping up, but I don't have a good way to develop confidence that the "reduced severity" list should genuinely be empty.

Maniphest Tasks: T13499

Differential Revision: https://secure.phabricator.com/D21044