Uses Twilio::REST::RestError when a page fails to load

[philnash]

This follows on from #391.

With the following code:

require './lib/twilio-ruby'

# We'll use a valid SID
account_sid = ENV['TWILIO_ACCOUNT_SID']
# But we'll use an invalid auth token
client = Twilio::REST::Client.new account_sid, "not_my_auth_token"
client.api.account.messages.list

the error message looks like:

/Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/framework/page.rb:32:in `process_response': Unable to fetch page (Twilio::REST::TwilioError)
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/framework/page.rb:22:in `initialize'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:212:in `initialize'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:181:in `new'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:181:in `page'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:127:in `stream'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:97:in `list'
	from test.rb:11:in `<main>'

But with this change looks like:

/Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/framework/page.rb:32:in `process_response': [HTTP 401] 20003 : Unable to fetch page (Twilio::REST::RestError)
Authenticate
Your AccountSid or AuthToken was incorrect.
https://www.twilio.com/docs/errors/20003

	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/framework/page.rb:22:in `initialize'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:212:in `initialize'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:181:in `new'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:181:in `page'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:127:in `stream'
	from /Users/pnash/projects/ruby-projects/twilio-ruby/lib/twilio-ruby/rest/api/v2010/account/message.rb:97:in `list'
	from test.rb:11:in `<main>'`

This relies on the work done in #391 as it really just uses the Twilio::REST::RestError in place of the Twilio::REST::TwilioError when it is raised in response to a non 200 status code in Twilio::REST::Page#process_response.

The rest of the work just ensures that anyone using the now defunct body method in the Twilio::REST::TwilioError doesn't end up raising NoMethodErrors instead. It no longer makes sense for Twilio::REST::TwilioError to have a body, but since it did before this change I don't want to break anything. Read more about the actual thought process for this in the comments to #391.

[philnash]

@efossier following up after #391. What do you think?

[codejudas]

+1 I like the improved messaging