Fix definition of ErrSystem type

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

errdefs/defs.go

@@ -35,7 +35,7 @@ type ErrForbidden interface {
 // ErrSystem signals that some internal error occurred.
 // An example of this would be a failed mount request.
 type ErrSystem interface {
-	ErrSystem()
+	System()
 }
 
 // ErrNotModified signals that an action can't be performed because it's already in the desired state

errdefs/helpers_test.go

@@ -179,3 +179,16 @@ func TestUnavailable(t *testing.T) {
 		t.Fatalf("causual should be errTest, got: %v", cause)
 	}
 }
+
+func TestSystem(t *testing.T) {
+	if IsSystem(errTest) {
+		t.Fatalf("did not expect system error, got %T", errTest)
+	}
+	e := System(errTest)
+	if !IsSystem(e) {
+		t.Fatalf("expected system error, got %T", e)
+	}
+	if cause := e.(causal).Cause(); cause != errTest {
+		t.Fatalf("causual should be errTest, got: %v", cause)
+	}
+}