window: Show an error dialog for invalid numbers

It’s better to inform the user why it failed instead of
silently failing

Fixes #381
8 jobs for wip/sadiq/inform-on-sms-fail in 13 minutes and 45 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Build
passed #306515
librem5
build-debian-gcc-bullseye

00:03:23

passed #306514
librem5
build-debian-gcc-buster

00:02:50

 
  Test
passed #306517
librem5
check-po

00:00:30

passed #306516
librem5
test:debian-gcc

00:01:55

21.1%
 
  Package
passed #306518
librem5
package:deb-debian-buster

00:01:49

passed #306519
librem5:arm64
package:deb-debian-buster:arm64

00:06:16

 
  Test Package
passed #306520
librem5
autopkgtest-debian-buster-package

00:01:14

passed #306521
librem5
lintian-debian-buster-package

00:00:36