execution: add shim runtime

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

execution/error.go

@@ -3,5 +3,8 @@ package execution
 import "fmt"
 
 var (
-	ErrProcessNotFound = fmt.Errorf("process not found")
+	ErrProcessNotFound   = fmt.Errorf("process not found")
+	ErrProcessNotExited  = fmt.Errorf("process has not exited")
+	ErrContainerNotFound = fmt.Errorf("container not found")
+	ErrContainerExists   = fmt.Errorf("container already exists")
 )