Avoid ensure-error for privacy reasons

Example:

```
> node
> const nonError = {foo: 'i-am-private'};
undefined

// before
> util.inspect(nonError);
'{ foo: \'i-am-private\' }'

// after
> nonError.toString()
'[object Object]'
>
```

js/modules/types/errors.js

@@ -1,7 +1,12 @@
-const ensureError = require('ensure-error');
-
 //      toLogFormat :: Error -> String
 exports.toLogFormat = (error) => {
-  const normalizedError = ensureError(error);
-  return normalizedError.stack;
+  if (!error) {
+    return error;
+  }
+
+  if (error && error.stack) {
+    return error.stack;
+  }
+
+  return error.toString();
 };