Ratul,
1. how do most operators configure their bgp (text editor or some high-level configuration tool)?
I think this varies a lot depending on the network and the type of configuration change being made. Some folks work directly at the command-line interface (perhaps with some cut-and-paste from previous configs, especially for boiler-plate stuff), some use GUIs provided by router vendors for some types of config changes, others use their own scripts, and still others might use third-party commercial products like Orchestream or Goldwiretech.
2. are there "configuration checkers" out there that would check whether an operator has not made a common error while modifying configuration?
For this, you might be interested in the paper http://www.research.att.com/~jrex/papers/ieeenet01.ps that appeared in IEEE Network Magazine in Sept/Oct 2001. The paper describes a checker that looks for inconsistencies within and across routers in an AS, by parsing and analyzing router configuration files. -- Jen