ipfs-webui: Improve UI when user tries to open missing file

(found this while working on unbundling webui from companion)

“Not Found” error for Files tab could be better 😃

1. Open http://127.0.0.1:8080/ipfs/QmXc9raDM1M5G5fpBnVyQ71vR4gbnskwnB9iMEzBuLgvoZ/#/files/I_AM_NOT_HERE.jpg (or any other filename that is not present in your MFS)

2. See this odd state with no buttons and bogus error about adding:

This may happen if someone bookmarks or copies URL to another machine without pointing webui at the same IPFS node.

We should either display a meaningful message or just silently redirect to the nearest MFS root.

### Tasks
- [ ] Display meaningful error message when file cannot be loaded
- [ ] Allow user to navigate back to paret files page easily