Make use of `msg` of `UniqueConstraint`
UniqueConstraint of YAMS (see
yams/constraints.py) can have a message (through parameter
BaseConstraint). If the constraint fails, we would like to have the given
msg if one given.
But we don't have this. Instead, the SQL error is given. It should be catched and replaced by
msg if its booleanity equals
The problem could seem to come from YAMS. But, YAMS does not do the SQL translation of the constraint, CubicWeb does (see
cubicweb/server/schema2sql.py). That is why we thought it is a CubicWeb issue.