Apply suggestions from code review

Author: fingolfin

src/ResidueField.jl

@@ -466,7 +466,7 @@ residue ring parent object is cached and returned for any subsequent calls
 to the constructor with the same base ring $R$ and element $a$.
 """
 function residue_field(R::Ring, a::RingElement; cached::Bool = true)
-   @req !is_trivial(R) "Zero rings are currently not supported as base ring."
+   @req !is_trivial(R) "Base ring must not be the zero ring."
    iszero(a) && throw(DivideError())
    T = elem_type(R)
    S = EuclideanRingResidueField{T}(R(a), cached)