Add more validation to AllValidatorsAreExited

[prestonvanloon]

What type of PR is this?

Other

What does this PR do? Why is it needed?

I think this method, which defaults to "true", can be error prone.
I have added a few checks:

• Return false when no keys are provided. "All keys are exited" cannot be true if there are no keys.
• Return false and an error if the server only partially responds to the request. The validator should not assume a key is exited if the server did not respond with a status for that key.

Which issues(s) does this PR fix?

Related: #7707

Other notes for review

I did not test at runtime.

This method is used here:

prysm/validator/client/runner.go

Lines 100 to 107 in 3485f3b

	allExited, err := v.AllValidatorsAreExited(ctx)
	if err != nil {
	log.WithError(err).Error("Could not check if validators are exited")
	}
	if allExited {
	log.Info("All validators are exited, no more work to perform...")
	continue
	}

In the event, that there is some erroneous failure, it will only log a message rather than stopping the active validators from performing work.

[codecov]

Codecov Report

Merging #7755 (ae1bb94) into master (742808c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #7755   +/-   ##
=======================================
  Coverage   62.16%   62.16%           
=======================================
  Files         429      429           
  Lines       30292    30296    +4     
=======================================
+ Hits        18831    18834    +3     
- Misses       8531     8532    +1     
  Partials     2930     2930