enhancement: remove panics and expects

[lostman]

Description

Closes #1098.

This PR cleans up remaining expects, unwraps and panics in the executor and service.

The only place where expects are still present is the Drop implementation of WasmArg. However, since WasmArg is created (and destroyed) in a spawn_blocking task, it will not affect the indexer service. If Drop were to panic!, the individual indexer would shut down (and would not be restarted automatically). The indexer service would be unaffected.

Testing steps

CI test should pass.

Changelog

• Added more error variants to WasmIndexerError. Changed expects and unwraps to returning Err variant which also triggers the early exit.
• Instead of panic!-ing or printing error! and break-ing the loop, the executor now returns an IndexerResult. The Err variant is printed when it reaches IndexerService run loop.