chore: PouchDB-find adapters/http update

[SourceR85]

This refactors dbFetch into an async function.
Instead of anonymous objects, instances of PouchError are now thrown.

Additional changes:

• update var to const.
• avoid setting "Content-Type" request header for getIndexes and deleteIndex.

I isolated this PR from #8860, because it includes type change of exceptions.

NOTE: Can't check for instanceof PouchError.
Class PouchError isn't exported, left a hints as FIXME in tests.