Improve compilation error handling

To avoid delays associated with retrieval of the stacktrace, highlighting is 
based on the exception message that is reported by the stderr-handler in 
nrepl-load-file-handler.  The exception trace is put in the *nrepl-error* 
buffer, but not displayed by default.

Adds an nrepl-visit-error-buffer function, to visit *nrepl-error*.

Adds nrepl-jump-to-compilation-error, bound to M-n, to jump to the
compilation error.

Adds an optional eval-error-handler to nrepl-make-response-handler.

Adds nrepl-popup-on-error, to control whether stacktraces (globally) are
popped up by default.