Add test for throwing error

src/exp.jl

@@ -73,8 +73,8 @@ function exponential!(A, method::ExpMethodNative, cache = nothing)
 end
 
 function exponential!(A::AbstractSparseArray, method=nothing, cache=nothing)
-    throw("exp(A) on a sparse matrix is generally dense. This operation is "*
+    throw(ErrorException("exp(A) on a sparse matrix is generally dense. This operation is "*
     "not allowed with exponential. If you wished to compute exp(At)*v, see expv. "*
     "Otherwise to override this error, densify the matrix before calling, "*
-    "i.e. exponential!(Matrix(A))")
+    "i.e. exponential!(Matrix(A))"))
 end

test/basictests.jl

@@ -99,7 +99,7 @@ end
 if VERSION >= v"1.9"
     @testset "exponential! sparse" begin
         A = sparse([1, 2, 1], [2, 1, 1], [1.0, 2.0, 3.0])
-        exponential!(copy(A), ExpMethodGeneric()) ≈ exp(Array(A))
+        @test_throws ErrorException exponential!(A)
     end
 end