cubicweb #3652306 UniqueTogetherError last arg has a trailing _idx [rejected]

In some cases with PG 9.2 at least, the index name in the pg error message has a trailing "_idx" that is not cleaned when preparing the UniqueTogetherError. This leads to cryptic messages returned to the end-user, with no mention of the actual field or relation that has to be fixed.

done in3.18.7
load left0.000
closed by<not specified>
patch[server] On constraint violation handling, clean the trailing _idx in index names. [rejected]