fix: non-string error responses are not treated as OpenIdConnectError

```
{ error:
      { code: 400,
        message: 'Request contains an invalid argument.',
        status: 'INVALID_ARGUMENT' } }
```
This is not an OpenIdConnectError and will no longer be returned as
such, this will now fall under throwing HTTPError instances instead.

See #125

lib/helpers/is_standard_body_error.js

@@ -2,7 +2,7 @@ module.exports = function isStandardBodyError(error) {
   if (error instanceof this.httpClient.HTTPError) {
     try {
       error.response.body = JSON.parse(error.response.body);
-      return !!error.response.body.error;
+      return typeof error.response.body.error === 'string' && error.response.body.error.length;
     } catch (err) {}
   }
 

test/issuer/discover_issuer.test.js

@@ -204,6 +204,20 @@ const fail = () => { throw new Error('expected promise to be rejected'); };
         });
     });
 
+    it('is rejected with HTTPError when error is not a string', function () {
+      nock('https://op.example.com', { allowUnmocked: true })
+        .get('/.well-known/openid-configuration')
+        .reply(400, {
+          error: {},
+          error_description: 'bad things are happening',
+        });
+
+      return Issuer.discover('https://op.example.com')
+        .then(fail, function (error) {
+          expect(error).to.be.an.instanceof(Issuer.httpClient.HTTPError);
+        });
+    });
+
     it('is rejected with when non 200 is returned', function () {
       nock('https://op.example.com', { allowUnmocked: true })
         .get('/.well-known/openid-configuration')