actually return errors

Just about every callback in the Node world returns a parameter called `error` to the callback function. `error` is expected to be an instance of some _Error_, so I tried to apply this expectation to this module:

``` js
var plotly = require('plotly')('kenany', 'blah');

var data = [{x: [0, 1, 2], y: [3, 2, 1], type: 'bar'}];
var graphOptions = {fileopt: 'extend', filename: 'nodenodenode'};

plotly.plot(data, graphOptions, function(error, msg) {
  if (error) {
    throw error;
  }

  console.log(msg);
});
```

When I do get an `error` above, though, what is thrown by the `throw` statement is a plain _Object_. This is completely against the expectation that `error` is, well, an _Error_.

Diving into this module's code, I discovered that this issue becomes even worse upon the realization that this module can return **4 different types** of errors:

  - [line 80](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L80) - _Object_ returned
  - [line 96](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L96) - _Error_ returned
  - [line 128](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L128) - _Object_ returned
  - [line 165](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L165) - _Error_ returned
  - [line 201](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L201) - **_Number_ returned?!**
  - [line 205](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L205) - **_String_ returned?!**
  - [line 209](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L209) - _Error_ returned
  - [line 255](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L225) - _Error_ returned
  - [line 247](https://github.com/plotly/plotly-nodejs/blob/a8affc38091d9d2d896437601004a252ea95ffdb/index.js#L247) - **_String_ returned?!**

So, even if I wanted to just deal with the fact that `error` above is an _Object_, I also have to account for the fact that `error` could be a _String_ or even a _Number_!

This patch makes it so that all `errors` are actually an instance of _Error_.