Dont set error_extending status on API extend call

Dont set error_extending status on API extend call

Recent change (Ifd523ac5e9039861cf87711dc5c4842b5cb524c2)
moved the quota check on extend_volume to the API layer
in order to provide feedback to caller if the quota check
failed.

The problem is that change still sets the volume status to
error_extending if the quota check fails.  There's no reason
to do this if we're now giving immediate feedback that the call
failed.

I'm also unclear on what the purpose of adding the reservations
to the rpc call are, but I'll double check on that before changing.

This patch leaves most of the previous change in place but
removes the setting of error_extending to the volumes status
on the quota fail.

Change-Id: I8521c876b8d5c344713f211fa0c8c0c58a12a879