Node should exit with a unique code on first slashing

[jakubgs]

Is your feature request related to a problem? Please describe.
Currently if a slashing occurs for one of the attached validators the node continues to run. I consider this behavior as inviting further slashings.

Describe the solution you'd like
The same way doppelganger detection returns a unique 129 exit code, so should the node the node exit - possibly with another code, like 130 - on first detected slashing. This is done in order to prevent any additional slashings.

Describe alternatives you've considered
None.

Additional context
The recent wave of slashings on mainnet that spanned a period of 10 hours shows that even bigger operators can fail to detect a slashing even in their setup and fail to prevent additional slashing if no engineer is available to stop the nodes.

I see no benefit in the node continuing to run after the first slashing, only great risk.