Mention errors.txt in CreatingPlugins

[warp-core]

I don't know what to call this.

From: https://github.com/endless-sky/endless-sky/issues/8449

Summary

Includes a short description of the errors.txt file in the CreatingPlugins wiki page. Also, rewrites the section about the plugin folder paths to specifically mention the "config" and "resource" directories to make it easier to refer to the location of the errors file as the "config" directory.

[SomeTroglodyte]

Simply launching from a shell will also show the errors, and is sometimes the faster way. This way also makes it clearer when there are zero errors.

[Zarkonnen]

Can you add the exact location where errors.txt can be found?

• Linux ~/.local/share/endless-sky/errors.txt
• Windows C:\Users\yourusername\AppData\Roaming\endless-sky\errors.txt
• macOS ~/Library/Application Support/endless-sky/errors.txt