C#: Add note about the class name in instantiate error

[raulsntos]

Adds a note about the requirement that a C# class name must match the script filename in which the they are defined to the instantiate error.

This should hopefully make the error more user-friendly and avoid issues such as #66419 in the future.

Feel free to suggest a better message, the goal is to mention the common reasons why the associated class was not found:

• File at the specified path does not exist.
  In case the user attached a script to a Node and then deleted the script without detaching it from the Node.
• The file exists but it contains no class or the class defined does not match the name of the filename.
• The script filename and the class name are case-sensitive.
  It seems common for users to define a class such as Player using PascalCase to follow .NET class naming conventions and then name the script player.cs because Godot uses snake_case for filenames by default.

[avilches]

One of the reason could be the assembly is copied in a different folder than the expected. Maybe something like: "Ensure the assembly is located in X" where X is the expected folder (csproj_folder.godot\mono\tem\bin or whatever). It could sound crazy, but It really happened to me because I had the main csproj folder in a different folder than the solution by mistake.

EDIT: or maybe this scenario (where csproj and godot.project are located in different folder) could throw an error in the dotnet build stage. I can open an issue/proposal with this...

[raulsntos]

@avilches It's no longer possible to move the .csproj, and if the built assemblies don't end up in the right place it would be a bug and not an user error.

If you still think there's a bug, make sure to test with beta 9 that was recently released and may fix it. If you can still reproduce it feel free to open a new issue with more details.