Sometimes a server might return a HTTP error 500 (this seems to happen
sometimes with Microsoft’s VSCode server, for example). Map this to
`G_IO_ERROR_HOST_UNREACHABLE` for now, which is a bit more specific than
returning `G_IO_ERROR_FAILED`, but without the hassle of introducing a
new public error domain which could give more detail.
In particular, this should allow gnome-software to show an error message
to the user for such failed downloads, rather than hiding the error and
logging the following:
```
not handling error failed for action download: While downloading http://packages.microsoft.com/repos/vscode/pool/main/c/code/code_1.45.1-1589445302_amd64.deb: Server returned status 500: Internal Server Error
```
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
(cherry picked from commit 6c79a57114)
740e4482c2