Returns null and does not cache when the source code of the script fails to load

[Rindbee]

This usually means that an ERR_FILE* error occurred.

Fix #76941.

Previously, using GDScriptCache::get_full_script() would ignore errors during loading. Now, all errors are not ignored.

Judging in which period the error occurred, it can be judged based on the return value:

1. null + err: Error during script loading (load_source_code()).
2. script + err: Error during script parsing.

[anvilfolk]

I'm not going to review because I am not confident of my knowledge of a lot of this meta-gdscript stuff, but I am a little wary of this approach!

My understanding is that GDScript tries pretty hard to not fail-fast, I think. If there's an error in parsing, we try to continue and parse as much as possible. I think that might be part of the reason why ResourceFormatLoaderGDScript::load() instantiates the script. Similarly, I think that's why we didn't return an empty script from get_shallow_script(). This might allow other scripts to attempt to analyze/compile despite other files having errors.

So I am particularly worried about changes in the user experience when people are writing code and causing certain errors as they edit. I wouldn't want half the project to halt/give errors because a single file currently has a parsing error.

But this is just my very limited understanding, so I would love to hear more from @vnen and @adamscott!

That said, I do think if there's an error, it ought to be reported by ResourceFormatLoaderGDScript::load(), as opposed to pretending there's no error by returning OK. That way the code can figure out what to do with that downstream, whether to handle it gracefully or throw an error itself, etc :)

[Rindbee]

Previously, errors during loading were ignored. Now, all errors are not ignored.

Judging in which period the error occurred, it can be judged based on the return value:

1. null + err: Error during script loading (load_source_code()).
2. script + err: Error during script parsing.

[adamscott]

Seems to do the job! Thanks @Rindbee!

[akien-mga]

Thanks!