Remove ErrorMessage

[colas31]

Following this issue #40,
I removed the ErrorMessage when no workspace are open.
I replace it by a welcome message in the three view.

Basically it makes no sense to display a message to the user when there is no workspace.
If we follow the VSCode recommendation, the notification message should:

• Respect the user's attention by only sending notifications when absolutely necessary

Lot of extension are working only when a workspace is open and no error message are displayed, they simply do anything, and are activated only when a workspace is open

[manifestinteractive]

This is such a great idea. Thanks for the contribution !!! I wanted the user to know they would not see anything and why, but this is a much better solution.

[colas31]

@manifestinteractive ,
I read your changes.
Using the setContext with several viewsWelcome is a really great idea. You can really reflect the state of the extension within a workspace open :)
Nice job

[manifestinteractive]

@colas31 thanks for the great idea !! I never even thought of using the content within the panel as a method to show the status of the panel when it was not showing the tree view. I am going to borrow that idea for a few other extensions I am working on ;)