refactor(electrum): remove unwrap()s and expect()s

[LagginTimes]

Partially resolves bitcoindevkit/bdk_wallet#30.

Description

This PR eliminates all unwrap() and expect() calls from bdk_electrum_client, replacing them with proper error handling. Given that all public methods already return Result, we now propagate error messages instead of panicking.

Changelog notice

• Removed all unwrap()s and expect()s from bdk_electrum_client.rs.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo +nightly fmt and cargo clippy before committing

[tnull]

Thank you for looking into this! One comment.

[evanlinjin]

I created a commit on top which cleans up some of the logic so that we return a more explicit error when we cannot find a point of agreement (a.k.a. wrong network): 7c1cc80. Feel free to cherry-pick this across if you find that it is appropriate.

I think it would make sense to include a sync-with-wrong-network test to make sure we hit this error.

[evanlinjin]

ACK b24ae6d