Add more accurate error message when an unexpected exception is raised

lib/espec/assertions/raise_exception.ex

@@ -72,7 +72,10 @@ defmodule ESpec.Assertions.RaiseException do
 
   defp error_message(subject, [module], err_module, positive) do
     if positive do
-      "Expected #{inspect subject} to raise the `#{module}` exception, but nothing was raised."
+      case err_module do
+        {false, nil} -> "Expected #{inspect subject} to raise the `#{module}` exception, but nothing was raised."
+        err_module -> "Expected #{inspect subject} to raise the `#{module}` exception, but `#{err_module}` was raised instead."
+      end
     else
       "Expected #{inspect subject} not to raise the `#{module}` exception, but the `#{err_module}` exception was raised."
     end

spec/assertions/raise_exception_spec.exs

@@ -5,6 +5,7 @@ defmodule ESpec.Assertions.RaiseExceptionSpec do
     let :func1, do: fn -> raise(ArithmeticError) end
     let :func2, do: fn -> 1 + 1 end
     let :func3, do: fn -> List.first(:a) end
+    let :func4, do: fn -> raise(ArgumentError) end
 
     context "Success" do
       it "checks success with `to`" do
@@ -67,6 +68,15 @@ defmodule ESpec.Assertions.RaiseExceptionSpec do
         end
       end
 
+      it "checks error with `to`" do
+        try do
+          expect(func4()).to raise_exception(ArithmeticError)
+        rescue
+          error in [ESpec.AssertionError] ->
+            expect(error.message) |> to(end_with "to raise the `Elixir.ArithmeticError` exception, but `Elixir.ArgumentError` was raised instead.")
+        end
+      end
+
       it "checks error with `to`" do
         try do
           expect(func2()).to raise_exception(ArithmeticError, "bad argument in arithmetic expression")