Refactor: Move Wallet errors into their dedicated file

[thunderbiscuit]

The src/wallet/mod.rs is currently a very big file (3200 lines exactly), and over the course of my work on #318 I noticed a few small refactors that could help keep things tidy (ok it's a drop in the bucket but still a win!) and reduce the line count a bit.

Notes to the reviewers

We had 3 wallet-related errors that were defined in the mod.rs file, but we also have a dedicated error.rs file which states:

//! Errors that can be thrown by the `Wallet`

The errors belong there in my opinion, and this is a small refactor moving them over.

Changelog notice

No changelog required.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

❌ Patch coverage is 0% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.46%. Comparing base (35502e0) to head (a9bfdbc).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/wallet/error.rs	0.00%	45 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #378   +/-   ##
=======================================
  Coverage   86.46%   86.46%           
=======================================
  Files          25       25           
  Lines        8527     8527           
=======================================
  Hits         7373     7373           
  Misses       1154     1154           

Flag	Coverage Δ
rust	86.46% <0.00%> (ø)

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[thunderbiscuit]

It's weird because it's a clean move of the types, no new code, but the codecov tool treats it as new code and finds it has 0% coverage.

[ValuedMammal]

Also we're not using ApplyBlockError at all it seems.

[ValuedMammal]

ACK a9bfdbc

I tested by replicating the move and found no significant differences.