#627 Handle errors consistently

#627 

Handle errors consistently and prevent many from showing at once (debatable)

The `WordPressAppDelegate` handled the prevention before with `[set/is]AlertRunning`. The WPError class has now taken all the error handling from the App delegate. The majority of the alerts have the 'Need Help?' button. A few alerts were left as UIAlertViews as they are 'user choice' (yes/no, or other buttons) type alerts and are not errors. Some alerts like local validation-triggered ones were also left without the 'Need Help?' button, as the error does not pertain to any network or WordPress.com problem.

What might be an issue and should be debated, is whether we want to prevent alerts from showing.

IMO we should never prevent an error, but instead cancel requests that depend on the failing requests in the queue. This is likely a lot more work than just preventing multiple alerts at the moment. The problem with preventing alerts from showing is if the users misses an important one (raised by @astralbodies)