moved isBuildslaveDir() to it's own module

Moved isBuildslaveDir() function from buildslave.scripts.runner module
to a new buildslave.scripts.base module. This way it is possible for to
move logic for checking that 'buildslave start' is run with valid
buildslave base directory inside buildslave.scripts.startup.start()
function.
changed error handling in buildslave.scripts.base.isBuildslaveDir()

 - Catch open/read errors while reading buildbot.tac, and print error
   message.
 - Print affected file and directory in error messages to ease
   troublshooting.
 - Moved all error messages inside isBuildslaveDir()