[GUROBI] Change error message for non-existent constraint

[staadecker]

This pull request fixes an issue where the following code

import pyoptinterface as poi
from pyoptinterface.gurobi import Model

m = Model()
x = m.add_variable()
constr = m.add_linear_constraint(x >= 1)
m.delete_constraint(constr)
m.get_constraint_attribute(constr, poi.ConstraintAttribute.Name)

outputs

RuntimeError: Variable does not exist

instead of

RuntimeError: Constraint does not exist