dh_gnome: Match version numbers that are plain integers

gnome-desktop version 43 is versioned 43, not 43.0. Previously this
failed to match the regular expression, resulting in generating
gnome:UpstreamVersion = "~" and gnome:NextUpstreamVersion = "1~",
and an uninstallable package.

Similarly, accept versions like 43+dfsg1 or 43~git20220920 as being
based on major version 43.

As a safety-catch against this happening again, make the script exit
unsuccessfully if it is unable to find the major version. If necessary
for a particular package, this can be bypassed by using
dh_gnome --no-gnome-versions.