Include context backtrace in message

[trowski]

PHP does not call __toString() on a previous exception, so our decision to wrap ContextPanicError in a ContextException had the unfortunate side-effect of removing the context trace from the output of an uncaught exception.

There's a couple ways we can approach this. This PR puts the trace as a string into the message. This has the side-effect of needing to create a stack-trace string for uncaught exceptions from Tasks, but is mostly negligible I'd think.

Another approach would be to use reflection to set the $trace property of the parent exception, forcing the child trace to be the trace of the ContextPanicError object.

(new \ReflectionProperty(parent::class, 'trace'))->setValue($this, $originalTrace);