[ESS] drop empty help buffers

[Sam Steingold]

when I do C-c C-v (ess-display-help-on-object) or "?mi" in *R*, I get
a help buffer with
==============
No documentation for 'mi' in specified packages and libraries:
you could try '??mi'
==============
I think it would be better if no new buffer were created and the error
message was shown in the echo area (aka minibuffer), just like when I do
M-x manual-entry RET zzz RET
I do not get an empty *Man* buffer but a message "Can't find the zzz manpage".

-- 
Sam Steingold (http://sds.podval.org/) on Ubuntu 11.10 (oneiric) X 11.0.11004000
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