Add useful error messages/types to Result<T, ()>

There are multiple functions that return Result<T, ()> in the code, this is equivalent to Option<T>, however instead of changing those to optional types, these should return useful error messages/types so it's easier to debug.

Example: https://github.com/graphprotocol/graph-node/blob/79390c39a13f166999ceea8defc55eedd05265ab/graph/src/data/subgraph/mod.rs#L169

Some of them could just return &str/String but preferably they should return a proper error type or just an instance of anyhow. If for any of them you decide to create new error types, consider using the crate thiserror 🙂

If you never used any of these crates mentioned above, take a look at our codebase, you'll find usages of both. And if you're doing something outside of the graph crate/folder, you might want to import those from it's prelude module.

Start with graph/src/data/subgraph/mod.rs at the linked example, then search the codebase for functions returning Result<T, ()>. Review existing usages of anyhow and thiserror, and map each candidate's failure cases before deciding on useful error messages or types. Done means the relevant unit-like results no longer discard error information across the identified code paths.