cubicweb #1777411 migration: schema modification commands shouldn't crash if already exists [resolved]

this is necessary because: * add_cube(s) is buggy * when replaying a failed migration, we don't want to think about what has already been added or not

Also, drop_* methods already have this behaviour, so this is more consistent.

prioritynormal
typebug
done in3.12.8
load0.200
load left0.000
closed by#867ec36530b8 [migration] closes #1777411: schema modification commands shouldn't crash if already exists