API returns inconsistent results for drop requests on non-existent databases

[smolse]

InfluxDB version: 1.3.1

When running drop continuous query request on non-existent database:

$ curl localhost:8086/query --data-urlencode 'q=drop continuous query "whatever" on "non_existent_db"'
{"results":[{"statement_id":0,"error":"database not found: non_existent_db"}]}

When running drop retention policy request on non-existent database:

$ curl localhost:8086/query --data-urlencode 'q=drop retention policy "whatever" on "non_existent_db"'
{"results":[{"statement_id":0}]}

Expected result would be to get error also for drop retention policy request.

[smolse]

I believe this was fixed in #8878.