save custom error objects to this.msg

[stuartf]

I commented in #207 about this. A project I work on is passing a custom error object like {code: 404, msg: 'Not Found'} as the message in our validators. The latest versions of validator break this by leaving this.msg as null when the message is an Object.

[chriso]

This PR breaks the unit tests introduced in #191.

The library interprets a message object as a mapping of validator methods to error messages.

check('foo@bar.com', {
    notNull: 'An email is required'
  , isEmail: 'The email is invalid'
}).notNull().isEmail();

Consider the following case

check('foo', {
    notNull: 'Expected something'
}).notNull().isInt()

When the isInt() validation fails, the library checks to see whether an error message was defined since an object containing validators => messages was specified. In this case, no isInt message was defined so it fails with the library default of Invalid integer. Your PR causes the library to instead fail with {notNull: 'Expected something'} as the error (old behaviour).

I can't see a way for both to coexist. Without reverting #191, I'd say the only way to customise the library so that it supports error objects is to patch your validator instance's check (or possibly Validator.prototype.check if you're not concerned about collisions).