rbs.rake: Skip to add tasks if rbs_rails is not able to load

[tk0miya]

rbs_rails is usually used in development mode. So it may be not able to load in the production mode. At present, the generated rbs.rake loads rbs_rails/rake_task. But it might fail in many projects.

This skips to add tasks if rbs_rails is not able to load.

Note: In your example, you also used a similar technique. https://github.com/pocke/rubykaigi-2023-lets-write-rbs/blob/master/rbs.rake#L1

[pocke]

The CI is failing, but it does not relate to your change. I'll fix this failure. Please wait a moment 🙏

[tk0miya]

:tada: Thank you for merging!