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

[LagginTimes]

Partially resolves https://github.com/bitcoindevkit/bdk_wallet/issues/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:

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