Improve lncli closechannel error messages for mempool fee and channel state (#9155)

[Suvrat1629]

Change Description

This PR addresses issue #9155 by making error messages for lncli closechannel more actionable and less confusing:

• When the mempool minimum fee is not met, the error now explains that a cooperative close transaction was created but not accepted, and suggests using lncli wallet bumpfee.
• If a channel is already in the process of closing, the error now clearly states this and guides users to check lncli pendingchannels and use bumpfee if needed. All error strings follow Go style guidelines.

Steps to Test

• Attempt to close a channel with a low fee and verify the new error message suggests bumpfee.
• Try to close a channel that is already closing and confirm the error directs you to pendingchannels and bumpfee.

Pull Request Checklist

Testing

• [ ] Your PR passes all CI checks.
• [ ] Tests covering the positive and negative (error paths) are included.
• [ ] Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• [ ] The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• [ ] The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• [ ] Commits follow the Ideal Git Commit Structure.
• [ ] Any new logging statements use an appropriate subsystem and logging level.
• [ ] Any new lncli commands have appropriate tags in the comments for the rpc in the proto file.
• [ ] There is a change description in the release notes, or [skip ci] in the commit message for small changes.

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[gemini-code-assist[bot]]

Summary of Changes

Hello @Suvrat1629, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the user experience for lncli closechannel by providing more actionable and less confusing error messages. It addresses common scenarios where channel closure fails due to insufficient transaction fees or when a channel is already in a closing state, guiding users with clear instructions on how to proceed and resolve these issues.

Highlights

• Improved Mempool Fee Error Message: The error message for lncli closechannel when the mempool minimum fee is not met now explicitly states that the cooperative close transaction was created but not accepted due to low fees, and suggests using lncli wallet bumpfee to resolve the issue.
• Enhanced Channel Closing State Error: If a channel is already in the process of closing, the error message now clearly informs the user of this state and guides them to check lncli pendingchannels for status and use lncli wallet bumpfee if the transaction is unconfirmed.

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[Suvrat1629]

However, I think the error wording could be improved.

Any suggestions on how to improve this? I tried to keep the message as direct as possible based on my understanding.

I would expect it would be good (not sure if it would be overkill) if you could also add a minimal itest for this.

I will add a test once you approve my changes. I think I need to add tests in rpcserver_test.go and btcwallet_test.go I presume? Could you please confirm.

Also, I have some thoughts on ErrMempoolFee, so I left my comment here: #9155 (comment)

Yes I did take a look at the comment and I am still learning about the codebase so I took a look at lnd/lnwallet/interface.go I think this is the base error and handles generic error messages, my change in the this pr handles the specific case mentioned in the issue.

[lightninglabs-deploy]

@nishantbansal2003: review reminder @suvrat1629, remember to re-request review from reviewers when ready