No error when try to open already opened exercise

[Alex18gr]

Description
No check if someone tries to open again an exercise that already exists in the workspace.

To Reproduce

1. Open an exercise
2. Try to open again the same exercise
3. No error apears and the exericse is not opened

Expected behavior
An error should appear that indicates that the exercise is already opened and exists in the workspace.

Environment

• OS: everywhere
• Eclipse Version 2021-03
• Version 0.0.8