[Security Solution] Show warning if rule contains any index patterns that match no indices

[nkhristinin]

Currently, if the rule has an index pattern with only non-existing indices - we show the warning:

existing index and a non-existing index

all privileges

• for users with all privileges (like default elastic user) - the rule will just succeed:

limited privileges

• for users with limited privileges - the rule will show a warning about lack of read privileges for non-existing index.

this warning is a bit misleading, and will be fixed in this PR

Proposal

But probably makes sense to show some sort of warning if the index pattern contains a non-existing index, as it can lead to confusion by the user.
if there was a typo in the index pattern, and the rule runs successfully but does not query some indices in the index pattern it can not generate results excepted from the rule, and it's hard to catch because there is no indication of that.

[elasticmachine]

Pinging @elastic/security-detection-engine (Team:Detection Engine)