Trait implemented by exception types that carry an optional payload.
Many ScalaTest events include an optional "payload" field that can be used
to pass information to a custom reporter. This trait facilitates such customization,
by allowing test code to include a payload in an exception (such as TestFailedException).
ScalaTest looks for this trait and fires any payloads it finds in the relevant ScalaTest event
stimulated by the exception, such as a TestFailed event stimulated by a TestFailedException.
(Although in its initial
release there is only two subclasses of PayloadField in ScalaTest,
in future version of ScalaTest, there could be more)
For an example of how payloads could be used, see the documentation for trait Payloads.
The optional payload.