FIX: properly close topics in vBulletin importer

FEATURE: add backtrace when an exception happen (importers)
FIX: post-processing should also happen on first posts (vBulletin
importer)
PERF: faster topic bypass when already imported

script/import_scripts/base.rb

@@ -402,6 +402,7 @@ class ImportScripts::Base
           skipped += 1
           puts "Exception while creating post #{import_id}. Skipping."
           puts e.message
+          puts e.backtrace.join("\n")
         end
       end