[8.x] Improve exception logging

[levacic]

I'm not sure if this should go to 8.x or master, let me know if I should switch the target branch.

Here's the commit message from the second commit (the first is just an aesthetic change to make the second commit more readable - feel free to squash if needed):

Add special handling when logging exceptions

• Illuminate\Foundation\Exceptions\Handler::report() already does this when logging caught exceptions
• This is well-defined behavior as per PSR-3

Previously, logged exceptions would be passed down to Illuminate\Monolog\Logger which forcibly casts them to strings, resulting in the __toString() method being called on them - which is inherited from Exception, which prints a stack trace. This stack trace ends up being used as the log message itself.

This change makes it so that when an exception is logged, the log message becomes a shorter and usually more readable string, which is the return value of the getMessage() method on the exception object, and passes the exception object itself as the exception key of the log message context, but only if that key wasn't already manually set by the user when logging the exception.

Monolog's LineFormatter will already process exceptions found in the context to include a nice stack trace, if the includeStacktraces option is turned on (which it is by default in Laravel).

This means that no information should be lost, unless someone is explicitly relying on the current behavior and/or making uncommon logging configuration changes.

Furthermore, this enables userland code to attach additional processors to the wrapped Monolog instance and include additional custom processing on the logged exception objects if desired.

Ref laravel/ideas#2449 (comment)

[levacic]

PSR-3 also specifies it's okay to pass objects as messages as long as they implement the __toString() method (which all exceptions do), and that implementors may have special handling for such messages, so this isn't unreasonable as a use-case which could be supported.

[taylorotwell]

We wouldn't change this on a patch release. Also would need to see before and after examples.

[levacic]

@taylorotwell Yeah wasn't sure how much of a BC break this would've been considered.

By before and after examples, do you just mean e.g. what gets output into laravel.log?