change the exception on invalid coverage mode

ext/datadog_cov/datadog_cov.c

@@ -107,7 +107,7 @@ static VALUE dd_cov_initialize(int argc, VALUE *argv, VALUE self)
   }
   else
   {
-    rb_raise(rb_eArgError, "mode is invalid");
+    rb_raise(rb_eArgError, "threading mode is invalid");
   }
 
   struct dd_cov_data *dd_cov_data;

spec/ddcov/ddcov_spec.rb

@@ -231,6 +231,16 @@ def thread_local_cov
             expect(coverage.keys).to include(absolute_path("calculator/operations/multiply.rb"))
           end
         end
+
+        context "when threading mode is invalid" do
+          let(:threading_mode) { :invalid_mode }
+
+          it "raises an error" do
+            expect { described_class.new(root: root, threading_mode: threading_mode) }.to(
+              raise_error(ArgumentError, "threading mode is invalid")
+            )
+          end
+        end
       end
     end
   end