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();
 };

test/modules/types/errors_test.js

@@ -9,36 +9,35 @@ const APP_ROOT_PATH = Path.join(__dirname, '..', '..', '..');
 
 describe('Errors', () => {
   describe('toLogFormat', () => {
-    it('should convert non-errors to errors', () => {
-      try {
-        // eslint-disable-next-line no-throw-literal
-        throw 'boom';
-      } catch (nonError) {
-        assert.typeOf(nonError, 'string');
-        assert.isUndefined(nonError.stack);
-
-        const formattedStack = Errors.toLogFormat(nonError);
-        assert.include(
-          formattedStack,
-          APP_ROOT_PATH,
-          'Formatted stack has app path'
-        );
-        return;
-      }
-
-      // eslint-disable-next-line no-unreachable
-      assert.fail('Expected error to be thrown.');
+    it('should return error stack trace if present', () => {
+      const error = new Error('boom');
+      assert.typeOf(error, 'Error');
+
+      const formattedError = Errors.toLogFormat(error);
+      assert.include(formattedError, 'errors_test.js');
+      assert.include(formattedError, APP_ROOT_PATH, 'Formatted stack has app path');
     });
 
-    it('should add stack to errors without one', () => {
+    it('should return error string representation if stack is missing', () => {
       const error = new Error('boom');
       error.stack = null;
       assert.typeOf(error, 'Error');
       assert.isNull(error.stack);
 
-      const formattedStack = Errors.toLogFormat(error);
-      assert.include(formattedStack, '<Original stack missing>');
-      assert.include(formattedStack, APP_ROOT_PATH, 'Formatted stack has app path');
+      const formattedError = Errors.toLogFormat(error);
+      assert.strictEqual(formattedError, 'Error: boom');
+    });
+
+    [
+      0,
+      false,
+      null,
+      undefined,
+    ].forEach((value) => {
+      it(`should return \`${value}\` argument`, () => {
+        const formattedNonError = Errors.toLogFormat(value);
+        assert.strictEqual(formattedNonError, value);
+      });
     });
   });
 });