Euclid preparation: VI. Verifying the performance of cosmic shear experiments by Euclid Collaboration, Paykari, P, Kitching, TD, Hoekstra, H, Azzollini, R, Cardone, VF, Cropper, M, Duncan, CAJ, Kannawadi, A, Miller, L, Aussel, H, Conti, IF, Auricchio, N, Baldi, M, Bardelli, S, Biviano, A, Bonino, D, Borsato, E, Bozzo, E, Branchini, E, Brau-Nogue, S, Brescia, M, Brinchmann, J, Burigana, C, Camera, S, Capobianco, V, Carbone, C, Carretero, J, Castander, FJ, Castellano, M, Cavuoti, S, Charles, Y, Cledassou, R, Colodro-Conde, C, Congedo, G, Conselice, C, Conversi, L, Copin, Y, Coupon, J, Courtois, HM, Da Silva, A, Dupac, X, Fabbian, G, Farrens, S, Ferreira, PG, Fosalba, P, Fourmanoit, N, Frailis, M, Fumana, M, Galeotta, S