wsi: Print error for unhandled bustype.

Currently there is no way for user to know that matching does not work
simply because it is not implemented. At least print an error.
13 jobs for !1596 with abort_match in 74 minutes and 6 seconds (queued for 20 seconds)
latest detached
Status Name Job ID Coverage
  Containers Build
passed debian #482319

00:00:20

 
  Build+Test
passed meson-arm64 #482328

00:07:13

passed meson-armhf #482327

00:04:01

passed meson-clang #482321

00:11:11

passed meson-clover #482325

00:10:48

passed meson-i386 #482329

00:03:36

passed meson-main #482324

00:09:30

passed meson-swr-glvnd #482320

00:03:25

passed meson-vulkan #482326

00:01:46

passed scons-llvm #482331

00:06:25

passed scons-nollvm #482330

00:05:18

passed scons-swr #482322

00:07:05

passed scons-win64 #482323

00:03:21