Add backtrace

Author: TonyCTHsu

lib/datadog/single_step_instrument.rb

@@ -8,5 +8,5 @@
 begin
   require_relative 'auto_instrument'
 rescue StandardError, LoadError => e
-  warn "Single step instrumentation failed: #{e.class}, #{e.message}"
+  warn "Single step instrumentation failed: #{e.class}:#{e.message}\n\tSource:\n\t#{Array(e.backtrace).join("\n\t")}"
 end