Detect 401 errors on opening a namespace

[paparent]

When clicking on a namespace on the left panel to open it, the
explorer should detect 401 errors (that's what happens if you don't
have LIST perm on the namespace). This can be seen when trying to
visit the dk.com namespace when not logged in (or logged in as someone
other than dk.com or dianas). The explorer currently displays the
spinning cursor indefinitely.

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Imported from Launchpad using lp2gh.

• date created: 2011-03-28T09:44:18Z
• owner: terrycojones
• the launchpad url was https://bugs.launchpad.net/bugs/744139