cubicweb #3153085 do not swallow IOError in bootstrap_cubes [in-progress]

For the except path concerns "non-cube" based tests (e.g. ext/rest stuff).
If the bootstrap_cube file is missing and one wants to run regular cubes tests, it is painful to find what's wrong.

prioritynormal
typebug
done in<not specified>
load0.200
load left0.200
closed by<not specified>
patch[devtools] do not swallow an IOError in bootstrap_cubes [rejected]