buildmaster and buildslave base directory checks

These patches (hopefully) improves error handling in isBuildmasterDir() and isBuildslaveDir().

* Catch open/read errors while reading buildbot.tac, and print error message, instead of exception Traceback
* Print affected file and directory in error messages to ease troublshooting.
* Print all error messages from isBuildmasterDir()/isBuildslaveDir()