Promote warnings to errors in array_push()

[Girgias]

Split from #4566

[nikic]

So, this is actually a pretty generic warning, in particular also used by $foo[] = $x, see

php-src/Zend/zend_execute.c

Line 1971 in 0038db2

zend_error(E_WARNING, "Cannot add element to the array as the next element is already occupied");

. I think whatever we do here, we should have a consistent behavior between $foo[] = $x and array_push($foo, $x).

[Girgias]

Appveyor failure seems unrelated.

[Girgias]

This probably needs to wait the result of the Reclassifying Engine Warnings RFC.

[cmb69]

That RFC has been accepted (this issue should trigger an Error exception), so we can proceed with this PR. :)

[nikic]

Merged as 1ca4ab0.