logilab-database #167781 drop_multicol_unique_index should not fail if the index does not exist in the database. [validation pending]

The method should not fail if the index does not exist. We want this in order to avoid, "under-the-hood" rollbacks in CubicWeb; see this ticket.

prioritynormal
typebug
done in1.13.4
load2.000
load left0.000
closed by#65e98f57d604 Drop indexes only if they exist (closes #167781).
patch[sqlserver] Drop unique index only if it exists [folded][generic] Drop indexes only if they exist (closes #167781). [applied][generic, sqlserver] Drop multicolumn unique indexes only if they exist (closes #167781). [folded]