Improve form validation in the editor plugin creation dialog

[t-karcher]

Godot version:
v3.2.4.beta4.official

OS/device including version:
Windows 10 64-Bit

Issue description:
I followed the official doc in a newly created project to create a plugin. After clicking "Create" in the "Create a Plugin" dialog, only the addons folder was created, but no subfolder and no files. In the debug window, there was the following error message:

ERROR: make_dir_recursive: Method failed. Returning: err
   At: core/os/dir_access.cpp:185

Steps to reproduce:

1. Create a new project with default settings.
2. In Project -> Project settings -> Plugins tab -> Click "Create"
3. Fill in the required fields and confirm with "Create"

Minimal reproduction project:

• Any newly created, blank project.

[Calinou]

What did you fill the plugin fields with before clicking OK and getting an error message? (Take a screenshot and upload it here.)

[t-karcher]

Ok, this is embarrassing. :-( I read the hint "my_plugin" --> res://addons/my_plugin the wrong way round and entered this:

Only after rereading the docs I noticed that just entering my_plugin and my_plugin.gd would have been enough. And with those values it works.

So yes, creation works, but validation of user input could be improved.

[Calinou]

So yes, creation works, but validation of user input could be improved.

I opened a pull request for that: #36677

There's also #41788.

[KoBeWi]

Resolved by #41788