Be less fragile and don't fall over updating or upgrading for one borked upstream.

[philclifford]

Working towards being less fragile so that one bad upstream change doesn't stop the well-behaved upstreams getting updates until we release fixes.

This needs to consider :

• break prettylist and we break cog
• Deborah <- need to check with @ymauray

currently listings and search bypass the failure so installed apps show up, but it would be better for users not to be presented with an app, then fail to try and install it because something (usually upstream changes ) broke it.

Options might include adding a flag/marker so that

• users can see a warning that a listed app cannot be installed
• csvilst users/apps can identify and filter "broken" apps

e.g.

• add an extra normally-null column for "broken" flag so apps can disable install/reinstall (but remove could be allowed, and show maybe too)
• add the same "broken" indicator to the "Pretty Name" (prepend or append, in glorious technicolor)

Input appreciated