VC shows stderr in target buffer

[David Reitter]

VC shows warnings from CVS in the target buffer, e.g. when invoking C- 
x C-v =, I would get things like

cvs diff: warning: failed to open /Users/dr/.cvspass for reading: No  
such file or directory

in the *vc-diff* buffer.  Wouldn't this be better shown in the echo  
area, if at all?
The warnings are quite a pain in the buffer, given that I would  
normally copy&paste the whole contents from there.

A quick look at vc.el suggests that `vc-do-command' calls `start- 
process', directing stdout and stderr into the current buffer.