Godot: 3.1-dev projects can't be opened on 3.0.x, should show a warning

[volzhs]

Godot version:

3.1.dev & 3.0.3-rc1

OS/device including version:

Kubuntu 18.04

Issue description:

3.0.x editor just restart project manager without warning message when opening 3.1.dev project.
It should say something.

Steps to reproduce:

1. Create a project with 3.1.dev editor
2. Try to open it with 3.0.x editor

Minimal reproduction project:

[vnen]

Probably because of the version bump in the Project Settings. Cc @groud

[groud]

Yeah maybe. Weird it does not displays a message.

[volzhs]

it should be warned that it can't be opened 3.0.x editor after opening it with 3.1.x, before opening.

[nobuyukinyuu]

You can fix this in the project.godot file by changing the version number back, but you'll lose your action binds when doing this, so be careful.

[aaronfranke]

A message such as "This project was opened in a future version of Godot and can't be opened by Godot [version]" would be more useful than just exiting with no message.

Also, when upgrading, a message such as "This project was previously opened in Godot [oldversion], would you like to update it to Godot [newversion]? This process is not reversible." would be nice.

[akien-mga]

Also, when upgrading, a message such as "This project was previously opened in Godot [oldversion], would you like to update it to Godot [newversion]? This process is not reversible." would be nice.

That part is #20626. I'll try to tackle both, even though it's tricky to show proper warnings at this early stage when the editor is not up yet.

[1F1R5T]

The project still exits itself without notification of any errors ..., here I am using the version of Godot_v3.1-beta2_x11.64

[akien-mga]

It's fixed on 3.1 onwards, we can't change 3.0.x releases :)

[Zireael07]

@akien-mga 1F1R51 says he's using 3.1 beta 2?

[akien-mga]

For creating the project, sure, but this issue is about opening 3.(n+1) projects in a 3.(n) release, so here it's about opening projects in 3.0.x.