modify error message [skip ci]

Although RootOf and roots can support EX domains, sorted
roots (as from real_roots and all_roots) does not. This limitation
guarantees that the roots will be orderable since decisions about
ordering of trivial roots (quadratic and binomial) will only depend
on determining the sign of Rational quantities.

>>> p
Poly(sqrt(2)*x**2 - x + 1, x, domain='EX')
>>> RootOf(p,0).n()
0.353553390593274 - 0.762959226424681*I
>>> RootOf(p,1).n()
0.353553390593274 + 0.762959226424681*I
>>> print filldedent(roots(p))

{sqrt(2)/4 + sqrt(2)*sqrt(-4*sqrt(2) + 1)/4: 1, sqrt(2)/4 -
sqrt(2)*sqrt(-4*sqrt(2) + 1)/4: 1}
>>> p.all_roots()
Traceback (most recent call last):
...