Open channel dialog does not show an error for already opened channels

[manuelwedler]

Describe the bug

If a user tries to open a channel a channel that already exists, the dialog does not prevent the user from submitting it. A Total deposit did not increase. error notification is shown. If the existing channel is in a closed state, a Channel is not in an open state. error notification is shown.

Expected behavior

The dialog should prevent the user from submitting if an open or closed channel already exists. Some error message should be shown in the dialog and the accept button should be disabled.