Fix mistaken dialog error message

[chapulina]

🦟 Bug fix

Summary

When closing Gazebo Sim's quick start dialog, you'll currently see an error message like this:

[GUI] [Err] [Dialog.cc:131] Failed to save file: /home/chapulina/.ignition/gazebo/gui.config.
Check file permissions.

Which is mistaken, because the file was in fact correctly chosen. The issue was treating the return value of SaveFile as a boolean instead of checking for the error code. See the SaveFile API.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸