Adds exception template variable available in custom error pages

[dbrumann]

Alongside status_code and status_text you also have access to exception template variable in custom error pages.

[wouterj]

If I look at the TwigErrorRenderer, this variable contains a flat PHP string that is generated by the HtmlErrorRenderer. Are we sure we get an exception object in this variable?

[dbrumann]

I am just playing around with it and I got an error when assuming it was a string:

[2020-02-22T09:49:00.650785+00:00] request.CRITICAL: Uncaught PHP Exception Error: "Object of class Symfony\Component\ErrorHandler\Exception\FlattenException could not be converted to string" at /Users/dbr/workspace/error-demo/var/cache/prod/twig/ac/ac9e7bd5dee35e014b7f05a2dc891d2edd36162209f88af6abe5aa1fb054ffb1.php line 69 {"exception":"[object] (Error(code: 0): Object of class Symfony\\Component\\ErrorHandler\\Exception\\FlattenException could not be converted to string at /Users/dbr/workspace/error-demo/var/cache/prod/twig/ac/ac9e7bd5dee35e014b7f05a2dc891d2edd36162209f88af6abe5aa1fb054ffb1.php:69)"} []

I will have another look into it, to make sure it wasn't a fluke.

[wouterj]

Nevermind, you're completely correct. The HtmlErrorRenderer returns $exception->setAsString(...), which does not return a string 🤦‍♂️ . Using fluent PHP methods in this way makes code really confusing to read imho.

Anyway, this PR is correct. 👍

[dbrumann]

I agree, it's a bit unfortunate. I will open an issue in symfony/symfony to straighten out the naming there and then update the docs accordingly.

[javiereguiluz]

Thanks Denis.