cli.py: Add '--force' arg to workspace_close

--force will skip confirmation prompts, and removing the workspace
you're currently using to load the project will fail without "--force".

buildstream/_frontend/cli.py

@@ -733,10 +733,12 @@ def workspace_open(app, no_checkout, force, track_, element, directory):
               help="Remove the path that contains the closed workspace")
 @click.option('--all', '-a', 'all_', default=False, is_flag=True,
               help="Close all open workspaces")
+@click.option('--force', '-f', default=False, is_flag=True,
+              help="Always close the workspace and/or delete your changes")
 @click.argument('elements', nargs=-1,
                 type=click.Path(readable=False))
 @click.pass_obj
-def workspace_close(app, remove_dir, all_, elements):
+def workspace_close(app, remove_dir, all_, force, elements):
     """Close a workspace"""
 
     if not (all_ or elements):
@@ -767,10 +769,13 @@ def workspace_close(app, remove_dir, all_, elements):
                     if not click.confirm('Are you sure you want to close this workspace?'):
                         click.echo('Aborting', err=True)
                         sys.exit(-1)
+                elif not force:
+                    raise AppError("Cannot close workspaces. Workspace {} is being used to load the project"
+                                   .format(element_name), reason='closing-required-workspace')
         if nonexisting:
             raise AppError("Workspace does not exist", detail="\n".join(nonexisting))
 
-        if app.interactive and remove_dir:
+        if app.interactive and remove_dir and not force:
             if not click.confirm('This will remove all your changes, are you sure?'):
                 click.echo('Aborting', err=True)
                 sys.exit(-1)