update-manager exits on dist-upgrade when tmp is mounted "noexec" and gives useless error message

Binary package hint: update-manager

Today I tried to upgrade my system from 9.04 to 9.10 RC via update-manager -d. When I clicked the button for the dist-upgrade, I got the release notes and then a dialog which sais it's downloading 2 files. After that, update-manager just crashed, without any notice.

TEST CASE:

Steps to reproduce:
run update-manager -d
click the "Upgrade" button in the top right.
click "Upgrade" in the dialog showing the release notes.
enter your password when asked for it.

What should happen:
the system is upgraded to the new distribution release, or a proper error message is shown.

What actually happens:
update-manager crashes.

When I run update-manager -d from the command line, this is the output:
$ update-manager -d
extracting 'karmic.tar.gz'
authenticate 'karmic.tar.gz' against 'karmic.tar.gz.gpg'
/var/lib/python-support/python2.6/dbus/connection.py:242: DeprecationWarning: object.__init__() takes no parameters
  super(Connection, self).__init__(*args, **kwargs)
sudo: unable to execute /tmp/tmpx8L0A4/karmic: Permission denied

Since it said something about permissions, I also tried to run it as root directly from the command line instead of entering my password when update-manager asks for it. This is the output:
$ sudo update-manager -d
extracting 'karmic.tar.gz'
authenticate 'karmic.tar.gz' against 'karmic.tar.gz.gpg'
/var/lib/python-support/python2.6/dbus/connection.py:242: DeprecationWarning: object.__init__() takes no parameters
  super(Connection, self).__init__(*args, **kwargs)
/usr/lib/python2.6/dist-packages/UpdateManager/DistUpgradeFetcher.py:79: GtkWarning: gtk_scrolled_window_add: assertion `bin->child == NULL' failed
  self.parent.scrolled_notes.add(textview_release_notes)
extracting 'karmic.tar.gz'
authenticate 'karmic.tar.gz' against 'karmic.tar.gz.gpg'

But now I also got an error message in the GUI:
Can not run the upgrade
This usually is caused by a system were /tmp is mounted noexec. Please remount without noexec and run the upgrade again.

This error message is right, I do have /tmp mounted with noexec. The problem is that it doesn't show up when I don't run update-manager as root, so many users will not know what's the problem when their upgrade fails.

ProblemType: Bug
Architecture: amd64
DistroRelease: Ubuntu 9.04
NonfreeKernelModules: nvidia
Package: update-manager 1:0.111.9
PackageArchitecture: all
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Uname: Linux 2.6.28-16-generic x86_64