gi hangs up if you try to open an already existing issue

It seems that `gi open` allows the (undocumented) syntax `gi open <issue-number>`, with following results:

`gi open 12345` or `gi open "12345"` (assuming no issue 12345 exists):
```
Created issue Not Found
There is no issue number Not.
```
No issue is being created then.

`gi open 123` or `gi open "123"` (assuming an issue 123 already exists): gi hangs up and the issue branch isn't being created or checked out.

Please throw an error instead (in all cases, when the issue title is a number)