Fix: Runner adds exceptions to instrumentation payload

[bestie]

The DataDog integration checks for :exception in the instrumentation payload but Runner does not add it.

Looks like an oversight to me. Should it be added?

[bestie]

Example of an exception check

racecar/lib/racecar/datadog.rb

Line 101 in b1b0574

if event.payload.key?(:exception)

[deepredsky]

there is no need to set the exception manually, we are using active supports notification mechanism that does this automatically for exceptions.