ConfiguredTargetFactory comments/logic wrongly suggest that a null ConfiguredTarget is always an error

For native rules like genquery which perform significant work at analysis time, the rule's `create` may return a null `ConfiguredTarget` without setting a ruleContext error - and this is not an error, but signals a skyframe restart.

In that case, the intent is that `ConfiguredTargetFunction.createRule` return null, we eventually reach ConfiguredTargetFunction.java:376 where the env.valuesMissing() check will apply, and ConfiguredTargetFactory.compute() will return a null signaling a restart.

This means the comments in ConfiguredTargetFunction.erroredConfiguredTarget are misleading, and the `return target != null ? target : erroredConfiguredTarget(ruleContext, null)` logic seen in `ConfiguredTargetFunction.createRule` might lead to failure or crash if we (1) allow analysis failures and (2) encounter a skyframe restart in genquery.

CC @brandjon