mm-account: Don't proceed if chat not found

And warn if that's the case,

Though this is not supposed to happen, this may happen if for some reasoon
the recipientlist is empty

Fixes #611
8 jobs for wip/sadiq/warn-if-no-chat-match in 14 minutes and 45 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-pureos-gcc-byzantium #376811
librem5

00:03:19

passed build-pureos-gcc-byzantium-no-purple #376810
librem5

00:02:36

 
  Test
passed check-po #376813
librem5

00:00:27

passed test:debian-gcc #376812
librem5

00:01:55

19.0%
 
  Package
passed package-pureos-byzantium #376814
librem5

00:01:52

passed package-pureos-byzantium:arm64 #376815
librem5:arm64

00:05:35

 
  Test Package
passed autopkgtest-pureos-byzantium-package #376816
librem5

00:01:24

passed lintian-pureos-byzantium-package #376817
librem5

00:00:37