importer: Show "Install Protocol Support" button when protocol support is missing

Currently, when the protocol support is not installed (e.g. kamera),
the importer will report "Unknown protocol" error and leave
the dialog empty without any further instruction. The behavior
is not friendly for 1-dot users as mentioned in the Nate's
blog article.

This commit adds a button named "Install protocol support for %1". The
button is only visible when `DocumentDirFinder` emits
`KIO::Error::ERR_CANNOT_CREATE_SLAVE` error. The button can

1. open Discover and directly open kamera page if the scheme is
   "camera".
2. open Discover and search the scheme for other schemes.

To make `DocumentDirFinder` able to emit error signal, a new signal
named `error()` is added to `DocumentDirFinder`.

FEATURE: 446420