spice up subject with short hash from error message

as we encountered some problems with the wrong tickets being reopened this change adds a short hash from the exception message to the subject of the redmine issues.

e.g.

/gems/actionpack-2.3.17/lib/action_view/paths.rb:74

raises:

"ActionView::MissingTemplate: Missing template a/_index.erb" or
"ActionView::MissingTemplate: Missing template b/_index.erb"

which are obviously different errors, but are considered to be the same as long as only the file where the error has been raised is the key.