empymod/empymod: Fix example: `tem_walktem`

Fix example tem_walktem, related to changes in scipy.quadrature: Replace not-existing private name scipy.integrate.quadrature._cached_roots_legendre with the public name scipy.special.roots_legendre. As a consequence of the above, changed in empymod the used, old name scipy.special.p_roots by new, more descriptive name scipy.special.roots_legendre. Improve publications-section in the example gallery. Change error reporting to native (instead of print(bla) and raise Error do raise Error(bla)); improve corresponding error testing by checking the error message too