Allow EditorImportPlugin to skip files in import()

[cgbeutler]

Describe the project you are working on

Json importer using gdscript's EditorImportPlugin

Describe the problem or limitation you are having in your project

I would like to be able to quietly bail on importing a file after doing a bit of parsing.
In my project, I first look to see if a script is provided in the import options. If it's not there then I read the json file and look for a 'script' member. If neither of those exist, I would like to just skip the file without any errors.
If there is a way to do this, I cannot find it anywhere. If I return OK without saving, I get the cryptic error: core/os/file_access.cpp:671 - Condition "!f" is true. Continuing. If I return any other error code, I get the "import failed" message. I cannot find a way to silently bail on importing a file short of writing a blank file to the .import folder (gross.)

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Allow users to skip a file from the import(...) function. (If this already exists, let me know and I will open a docs proposal instead.)
I would propose one of two solutions: Allow the returning of OK without saving to "skip" a file. Allow the returning of ERR_SKIP to skip importing a file.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Example usage:

func import(source_file, save_path, options, r_platform_variants, r_gen_files) -> int:#Error
  
  ... #read and parse json file return real errors if those fail
  
  # If it is a valid json, but doesn't have our script variable, then silently bail on the file
  if not json.has("script"):
    return ERR_SKIP
  
  ... # save stuff
  
  return OK

I don't know enough about the back-end to comment there. The new "Keep File (No Import)" option could maybe be some help, but I don't actually know.

If this enhancement will not be used often, can it be worked around with a few lines of script?

An error can be returned and the console will just be spammed each time out leave and re-focus the editor. Not ideal, but things do keep working.

Is there a reason why this should be core and not an add-on in the asset library?

I think the code in question is in core.

[cgbeutler]

Tracked down what looks like the a possible spot for this. Checking for "ERR_SKIP" before checking for "OK" would maybe allow you to skip printing errors:
https://github.com/godotengine/godot/blob/c7b97f077a0f62d9e280adbddddc901f35e4f40f/editor/editor_file_system.cpp#L1781-L1783

As for the md5 error printed out if you return OK without saving anything, looks like dest_paths get filled with what it thinks will be the path. Then later the md5's are generated, giving an error, since that path doesn't exist. This section does skip dest_paths if the extension is empty. Would need to also skip when it gets ERR_SKIP or something.
https://github.com/godotengine/godot/blob/c7b97f077a0f62d9e280adbddddc901f35e4f40f/editor/editor_file_system.cpp#L1802-L1823
If not that, then the md5 section later could check for the file first or something. Regardless, that code seems like a decent entry point to figuring it out.