Mention error_string() in the Error enum docs

**Your Godot version:** 4.4

**Issue description:**

I found the Error enum from following a link in the editor. Trying to search the web or the docs site for "error" was much less helpful. So it took me a while (and one partially written Discord help thread) before I searched in page and found [error_string()](https://docs.godotengine.org/en/stable/classes/class_@globalscope.html#class-globalscope-method-error-string). I think it would be useful to mention and link to this under the Error enum docs.

Absent an obvious solution, seeing Error was an enum sent me on a wild goose chase trying to figure out how to look up enum names (and finding examples using keys() which doesn't work anymore)

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/classes/class_@globalscope.html#enum-globalscope-error

Start at the Error enum section on the linked GlobalScope documentation page and inspect the surrounding reStructuredText source. Add a mention and link for error_string() there, then build or preview the documentation to confirm the link and wording appear under the Error enum docs.

Written by the indexing model from the issue text.