[FR] verify db existence before asking for password

**As-is:**
- $  keepmenu -d /data/does-not-exist.kdbx
- \<password is asked>
- `Database does not exist` error is returned

**To-be:**
- $  keepmenu -d /data/does-not-exist.kdbx
- `Database does not exist` error is returned, password is not queried