Merge pull request #3240 from DataDog/tonycthsu/fix-missing-variable-…

…for-host-injection

Fix host injection error handling

lib-injection/host_inject.rb

@@ -113,5 +113,5 @@ def debug_log(msg)
     end
   end
 rescue Exception => e
-  warn "[ddtrace] #{failure_prefix} #{e.class.name} #{e.message}\nBacktrace: #{e.backtrace.join("\n")}\n#{support_message}"
+  warn "[ddtrace] Injection failed: #{e.class.name} #{e.message}\nBacktrace: #{e.backtrace.join("\n")}\n#{support_message}"
 end