feat(error): add stable error hierarchy

examples/error.rb

@@ -0,0 +1,12 @@
+#!/usr/bin/env ruby
+
+require_relative "../lib/wreq"
+
+begin
+  Wreq.get("not-a-valid-url")
+rescue Wreq::Error => error
+  puts "#{error.class}: #{error.message}"
+  puts "builder: #{error.builder?}"
+  puts "uri: #{error.uri.inspect}"
+  puts "status: #{error.status.inspect}"
+end