Currently a server-side error (like a bad permission) produces the following
error on the client side when cloning by http (apache server) :
abort: consistency error adding group!
transaction abort!

Such permissions error may not be an uncommon error and as such, it would be
good to raise some informative error on the client side. 
It may perhaps be possible, when  RevlogError(_("consistency error adding
group")) is raised to have a look at the incoming chunk of data to check if the
client get a backtrace, and then inform the user...