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.