Bind C-c ` to cider-visit-error-buffer

Hi,

I am a bit disconserned by the current behavior of stack-trace auto popping I
used to disable `cider-popup-on-error` and call `cider-visit-error-buffer`
whenever I needed it.

In cider-dev cider-popup-on-error is not used anymore, and as a result there is
no way to generate stacktrace in the background. When `cider-popup-stacktraces`
is set to nil, stacktrace buffer is not generated at all. I think this is a bug.

Anyways, the proposed patch is a first step towards the user pattern I have just
described. I think more people might find it useful. Even if stacktrace pops on
error, <kbd>C-c `</kbd> is still useful when the stacktrace has been covered for
whatever reason.

Thanks.