Add RequestError typescript definition (#248)

Errors triggered by nano are extended with additional fields, this commits adds
the typescript definition for them.

The name "RequestError" was chosen because it seems like it all nano errors are
triggered by a request (a 'socket' or a 'couch' error).

I considered "NanoError", but the full type `nano.NanoError` doesn't sound
better than `nano.RequestError` and no other interface have been prefixed with
"Nano".

Author: j3k0

lib/nano.d.ts

@@ -36,7 +36,31 @@ declare namespace nano {
     request?(params: any): void;
   }
 
-  type Callback<R> = (error: any, response: R, headers?: any) => void;
+  type Callback<R> = (error: RequestError | null, response: R, headers?: any) => void;
+
+  // An error triggered by nano
+  interface RequestError extends Error {
+    // An error code.
+    error?: string; // 'not_found', 'file_exists'
+    // Human readable reason for the error.
+    reason?: string; // 'missing', 'The database could not be created, the file already exists.';
+    // Was the problem at the socket or couch level
+    scope?: 'couch'  | 'socket';
+    // Status code returned by the server
+    statusCode?: number; // 404;
+    // Request sent to Couch
+    request?: {
+      method?: 'GET' | 'POST' | 'PUT' | 'DELETE';
+      headers?: { [key: string]: string | number };
+      uri?: string; // 'http://couchhost:5984/db/tsp2',
+      qs?: any; // { revs_info: true }
+    };
+    // Response headers
+    headers?: { [key: string]: string | number };
+    // Error identifier
+    errid?: 'non_200' | 'request'; // string; // 'non_200'
+    description?: string;
+  }
 
   interface ServerScope {
     readonly config: ServerConfig;