[Reportbug-maint] Bug#894916: reportbug: print "Please install python3-urwid" instead of "Ignored bogus setting for --ui: urwid"

[Paul Wise]

Package: reportbug
Version: 7.1.10
Severity: minor

When I tried the urwid UI for reportbug I got an error message:

$ reportbug --ui urwid
Ignored bogus setting for --ui: urwid

After installing python3-urwid I got the urwid UI.

I think it would be nicer to get this error message instead:

$ reportbug --ui urwid
Please install python3-urwid to use the urwid UI.

-- System Information:
Debian Release: buster/sid
APT prefers testing-debug
APT policy: (900, 'testing-debug'), (900, 'testing'), (800, 'unstable-debug'), (800, 'unstable'), (790, 'buildd-unstable'), (700, 'experimental-debug'), (700, 'experimental'), (690, 'buildd-experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.15.0-2-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_AU.utf8, LC_CTYPE=en_AU.utf8 (charmap=UTF-8), LANGUAGE=en_AU.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages reportbug depends on:
ii apt 1.6~beta1
ii python3 3.6.4-1
ii python3-reportbug 7.1.10
ii sensible-utils 0.0.12

reportbug recommends no packages.

Versions of packages reportbug suggests:
pn claws-mail <none>
ii debconf-utils 1.5.66
ii debsums 2.2.2
pn dlocate <none>
pn emacs24-bin-common | emacs25-bin-common <none>
ii exim4 4.90.1-5
ii exim4-daemon-light [mail-transport-agent] 4.90.1-5
ii file 1:5.32-2
ii gir1.2-gtk-3.0 3.22.29-2
ii gir1.2-vte-2.91 0.52.0-1
ii gnupg 2.2.5-1
ii python3-gi 3.28.1-1
ii python3-gi-cairo 3.28.1-1
pn python3-gtkspellcheck <none>
pn python3-urwid <none>
ii xdg-utils 1.1.2-2

Versions of packages python3-reportbug depends on:
ii apt 1.6~beta1
ii file 1:5.32-2
ii python3 3.6.4-1
ii python3-apt 1.6.0~rc2
ii python3-debian 0.1.32
ii python3-debianbts 2.7.2
ii python3-requests 2.18.4-2

python3-reportbug suggests no packages.

-- no debconf information