Update tests for error messages

Author: eschnett

test/reduce.jl

@@ -251,7 +251,7 @@ prod2(itr) = invoke(prod, Tuple{Any}, itr)
 
 # maximum & minimum & extrema
 
-@test_throws "reducing over an empty" maximum(Int[])
+@test_throws "reducing with maximum over an empty" maximum(Int[])
 @test_throws "reducing over an empty" minimum(Int[])
 @test_throws "reducing over an empty" extrema(Int[])
 

test/reducedim.jl

@@ -184,7 +184,8 @@ end
     @test prod(A) === 1
     @test_throws ["reducing over an empty",
                   "consider supplying `init`"] minimum(A)
-    @test_throws "consider supplying `init`" maximum(A)
+    @test_throws ["reducing with maximum over an empty",
+                  "prevent this error by supplying an `init`"] maximum(A)
 
     @test isequal(sum(A, dims=1), zeros(Int, 1, 1))
     @test isequal(sum(A, dims=2), zeros(Int, 0, 1))