ninja backend: do not fatally error on compdb failure
ninja backend: do not fatally error on compdb failure

We print a warning if a compilation database isn't successfully
generated, which is good, because that gives some visibility in case the
user really wanted to use the compdb. But warnings default to being
fatal with --fatal-meson-warnings, which is not so good, because this
isn't a very important warning at all, and we'd rather not error out in
such cases when building works fine and a random bonus IDE feature
doesn't work.

Mark this particular warning as non-fatal.

Fixes side issue in https://github.com/mesonbuild/wrapdb/pull/343#issuecomment-1074545609

mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson