window: Show an error message if creating SMS with modem missing

Currently if the user tries to create an SMS with no supported
modem, chatty silently fails.  Let’s show an error dialog so that
the user knows why SMS conversation wasn’t created.

Fixes Librem5/chatty#313
7 jobs for wip/sadiq/modem-error-dialog in 11 minutes and 54 seconds
latest
Status Job ID Name Coverage
  Build
passed #266636
librem5
build-debian-gcc-bullseye

00:03:36

passed #266635
librem5
build-debian-gcc-buster

00:03:24

 
  Test
passed #266637
librem5
test:debian-gcc

00:02:15

 
  Package
passed #266638
librem5
package:deb-debian-buster

00:01:51

passed #266639
librem5:arm64
package:deb-debian-buster:arm64

00:04:28

 
  Test Package
passed #266640
librem5
autopkgtest-debian-buster-package

00:01:30

passed #266641
librem5
lintian-debian-buster-package

00:00:49