Clarify hydra-notify eval_failed

[Rowan-Mather]

Is your feature request related to a problem? Please describe.
From the manual entry on hydra-notify, my understanding was that an event for eval_failed is triggered when you get an evaluation error such as the one below. But this does not seem to be the case.

Perhaps this is only triggered when hydra cannot perform an evaluation of any description - irrelevant of whether it results in an evaluation error or not?

Example evaluation error from the #tabs-errors page:

evaluation failed with exit code 255

Describe the solution you'd like
Please be specific in the manual.