lightning precheck message "smaller number for tikv-importer.disk-quota(8EiB)" too ambiguous when local disk space check fails

[fubinzh]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Use lightning to import data where lightning local disk space is not enough.

2. What did you expect to see? (Required)

Precheck fail message should be clear, and be able to guide use to fix the issue

3. What did you see instead (Required)

1. "imprtestimate" should be separated as 2 words
2. "You need a smaller number for tikv-importer.disk-quota(8EiB) to finish imports" is confusing, user doesn't know how to fix the issue, smaller number than 8EiB is too ambiguous.

4. What is your TiDB version? (Required)

Release Version: v5.4.0-nightly
Git Commit Hash: 732b7eb
Git Branch: heads/refs/tags/v5.4.0-nightly
Go Version: go1.16.4
UTC Build Time: 2021-12-02 08:08:09
Race Enabled: false

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.