Drop AJAX error detection helpers

I feel like the error detection functions that we provide aren’t all that useful. Specifically:

• Functions should generally take one kind of input and return one kind of output. We take either an AjaxError instance or a number and try to verify that it’s describing some kind of error. While useful, I think this is kind of confusing behavior.
• People really can’t (or shouldn’t) have code where the argument is some variable that could be either an error object or a number; they must know ahead of time whether it’s a number or error object and could just check it directly
• If they are using it with only one argument type in mind, then they should just do a number comparison, or an instanceof check, themselves. Preferably the latter, since our descriptive error classes remove the need checking the status code directly.

If we were to remove them in v3.0.0, then for the time being we would deprecate the error checkers, and then remove them entirely in v3.0.0.

[alexlafroscia]

I’ve changed my mind on this and would rather just keep them around.

[sandstrom]

@alexlafroscia Done! https://github.com/ember-cli/ember-ajax/issues/292

I don’t think Ember.Error is reopenable (not in the same way other Ember Objects are, anyway). But yes, reopenable would definitely work.