Add a little error handling for invalid auth

When changing my api keys, realized I couldn't reauth. Now on a bad
auth, we clear the keychain so that we can reauth the next time

FitKit/FitbitClient.swift

@@ -168,6 +168,7 @@ class FitbitClient {
 
 
         Alamofire.request(urlString)
+            .validate()
             .validate(contentType: ["application/json"])
             .responseJSON(completionHandler: handler)
     }

FitKit/ViewController.swift

@@ -88,8 +88,18 @@ class ViewController: UIViewController {
             response in FitbitClient.weightResponseHandler(response: response) {
                 fbWeights, error in
                 if let error = error {
-                    NSLog("Unable to get weights: \(error)")
+                    NSLog("Unable to get weights: \(error.localizedDescription)")
+                    if let error = error as? AFError {
+                        if let suggestion = error.recoverySuggestion {
+                            NSLog("Suggestion to resolve: \(suggestion)")
+                        }
+                        if error.responseCode == 401 {
+                            NSLog("Unauthorized. Clearing credentials")
+                            self.client.clearCredential()
+                        }
+                    }
                     self.outputText.text = "Error getting weights"
+
                     return
                 }