Rollup Action must throw exception when rollup index already exists

[talevy]

Currently, the Rollup Action defined in #64900 does not have safeguards against the API running on anything other than an existing-non-rollup-index as the source index, and a rollup-index that does not exist yet.

If rollup index already exists, an exception should be thrown.

relates #42720.

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)