Add UserError WasmIndexerError code and extract an error message from WASM

[lostman]

Description

Similarly to what was done in #1402 with Panic and panic_hook, we should be able to change the indexer's handler functions' types to -> anyhow::Result<()>, and generate a wrapper which converts the Err variant into WasmIndexerError::UserError, while storing the message in a static mut String to be extracted and logged by fuel-indexer.

This would give the users another method of reporting errors, using ?. However, due to error location attribution, panicking may be preferable.

It could also be helpful for #1316.