Add more info alongside failed steps in messages to inform about the failure itself

[aurelien-reeves]

Refs. cucumber/cucumber-js#1453

Is your feature request related to a problem? Please describe.

The idea came from the discussion in an issue opened for cucumber-js: cucumber/cucumber-js#1453
It may help third-party developers to have more info regarding failures in the message testStepFinished.

Info which have been identified as nice to have:

• expected
• actual
• error message (it is already, as "message" I guess?)

Would those info be easy to retrieve from our various implementation?

The issue on cucumber-js also request for the error object itself, but that may be more difficult (impossible?) to properly serialize it in a message.

[mpkorstanje]

Perhaps also a stacktrace?

For Java this is relatively easy to support this for testing libraries that use opentest4j.

https://github.com/ota4j-team/opentest4j/blob/master/src/main/java/org/opentest4j/AssertionFailedError.java

[aurelien-reeves]

I though about it a stacktrace too, yes.
Extract it from the message may be an interesting improvement and simplify working with failed steps later

[davidjgoss]

This has some overlap with #69