Fix #194 - stacktrace buffer was not respecting nrepl-popup-stacktraces

In `nrepl-default-err-handler` we were checking only if the major mode is
nrepl-mode which caused the `nrepl-popup-stacktraces` setting to be
ignored in clojure-mode. As a bonus I've simplified a lot the code
that was used to check the buffer's major mode.

This fixes issue #194.