Enable intl.use_exceptions in debug mode

[nicolas-grekas]

Q	A
Branch?	6.2
Bug fix?	no
New feature?	no
Deprecations?	no
Tickets	-
License	MIT
Doc PR	-

Let's be stricter by default in debug mode.

[nicolas-grekas]

Hum, not sure about this on second thought: when enabled in dev, ppl will write try/catch. But in prod, intl will likely behave with no exceptions and the code should be written differently to handle that. The correct way to use intl in a portable way is to handle both situations, like we did in #36149
But there is no way to force ppl to handle both cases so most of the time they will stick to one mode, and then, prod&dev should be aligned.
PHP'd better deprecate this ini setting altogether and stick to one and only one mode.