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()
changed error handling in buildslave.scripts.base.isBuildmasterDir()

     - 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 isBuildmasterDir()