[Debug] Support any Throwable object in FlattenException

[javiereguiluz]

See symfony/symfony#26528

@derrabus you created the original pull request. I'd like to ask you if this change is just internal to Symfony or has impact to end-user apps. If it impacts users, can you please tell us what changes must users make? Thanks a lot!

[derrabus]

@javiereguiluz Thanks for reaching out. I will have a look if the documentation has to be adjusted. But since the old methods have not (yet?) been deprecated, there is no action developers have to take now.

The changes that end-users might actually see have been done in symfony/symfony#26028 already.