Fix misleading error message in setup_repo.rs gitreview.username configuration

[coderabbitai]

Description

The error message at line 45 in rs/src/modules/setup_repo.rs is misleading. It says "git-review username not set!" but it's actually reading from config.git_username.

This could confuse users trying to fix their configuration. The error message should either be corrected to reference "Git username" or a separate gitreview_username field should be used if that's the intended design.

References

• PR: Rewrite the project in Rust #49
• Comment: Rewrite the project in Rust #49 (comment)
• Reported by: @SomeMWDev