Tweak exception logging

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7972, #7973, #7976
| License       | MIT
| Doc PR        | n/a

This seems to be all it takes for the three issues. As of Monolog 1.5, exceptions passed in the log context will be dumped including any previous exceptions.