server: Don't assert when we fail to initialize the backend

So far we asserted which confuses users thinking that there's someting
wrong in phoc when usually there's something wrong with their EGL or DRM
setup.

The solution is not ideal since the servier is a singleton and hence
we need to evaulate the error in `phoc_server_get_default ()` but since
it stays around forever that doesn't hurt much atm.
14 jobs for dont-assert in 11 minutes and 39 seconds (queued for 1 minute and 49 seconds)
latest
Status Job ID Name Coverage
  Build
passed #359048
librem5
build-with-xwayland-alpinelinux-edge

00:00:45

passed #359046
librem5
build-with-xwayland-debian-bullseye

00:02:02

passed #359045
librem5
build-with-xwayland-debian-buster

00:02:06

passed #359049
librem5
build-without-xwayland-alpinelinux-edge

00:00:46

passed #359047
librem5
build-without-xwayland-debian-buster

00:02:03

 
  Test
passed #359050
librem5
unit-test-with-xwayland-debian-buster

00:01:53

43.1%
 
  Package
passed #359051
librem5
package-pureos-amber

00:01:30

passed #359052
librem5:arm64
package-pureos-amber:arm64

00:04:08

passed #359053
librem5
package-pureos-byzantium

00:01:32

passed #359054
librem5:arm64
package-pureos-byzantium:arm64

00:04:12

 
  Test Package
passed #359055
librem5
autopkgtest-pureos-amber-package

00:01:03

passed #359057
librem5
autopkgtest-pureos-byzantium-package

00:01:12

passed #359056
librem5
lintian-pureos-amber-package

00:00:39

passed #359058
librem5
lintian-pureos-byzantium-package

00:00:42