Specify that caught exceptions should be assigned to a variable named `$e`

320 out of 372 catches in the symfony code base use `$e`.

````
git grep "catch (.*Exception" | grep "\$e)" | wc -l                                                                                                                                               
320
git grep "catch (.*Exception" | wc -l
372
````

I think it would be good to make this a standard?