HTTP API return 404 error in different format than other endpoints

#### Version information:

0.4.5-pre1.

#### Type:

Bug

#### Priority:

P3?

#### Description:

When the go-ipfs HTTP API is queried and an error occurs during the query, go-ipfs returns the error in format `{ Code: 500, Message: "Omg!" }` (JSON). However, when the requested endpoint is not found and a 404 is returned, the error is not in JSON but a string.

This is annoying because we have to handle 404 separately from other errors in js-ipfs-api. It'd be great if all errors were returned in the same format.

Perhaps set the content-type header for 404 to 'application/json'.