WARNING: Breaking changes to the ABI and API regarding the Error class

Warning: commit `dfe4a94` breaks build on PHP7.0 and PHP7.1. Needs fix before proper release

Renamed exception to throwable
Enumeration Php::Error has been renamed to Php::ErrorType to make room for the new Php::Error class
Added new class Php::Error that can be used for throwing errors (PHP7 normally throws errors instead of reporting fatal errors, which is what PHP5 did)
Php::Exception is now only used for exceptions and no longer for errors (so extensions can be written to only catch exceptions, and not the errors)
Removed support for Exception::file() and Exception::line()
A couple of functions that used to report fatal errors, now throw an Php::Error object instead
fixed exception handling for class methods and functions (uncaught Error objects caused a full crash, now they cause a fatal error)
when calling an invalid function we no longer throw an exception, but an error
renamed ErrorType to Message
closures now get an empty string as name because exception handling functions sometimes need access to the name
fixed issue when dealing with optional object parameters