Feature request: use a unique Error type for operational errors

I think it's great that superagent embraces the Error object, but it would be cool if there were an easy way to tell if an error originated from Superagent. This is useful for standardizing error reporting in an application: for example, I can do something like:

```
if (error instanceof SuperagentError) {
// collect information from error object and log it somewhere
}
```

Of course, I can do duck-typing and look for properties like `timeout` or `response` on the error, but this introduces a conflict with other errors that might use those properties.

My suggestion: operational errors should be of type `SuperagentError` rather than `Error`. This type should be exported in the package for easy comparison (and set on the global object for people not using a module loader).

Happy to write a PR if this is a desired feature.