unit test config.ConfigErrors.__str__() method

Added unit test on config.ConfigErrors.__str__() method
unit test configure file open error in MasterConfig.loadConfig()

Added a unit test that simulates IOError when opening configure file in
config.MasterConfig.loadConfig(). Check that the expected ConfigErrors
exception is rised.
removed unreachable code in config.MasterConfig.loadConfig()

The call to error() with global _errors set to None, will raise
an exception. Thus following 'raise errors' is never reached.
unit test on loading 'codebaseGenerator' master parameter

Added unit tests on code in buildbot.config.MasterConfig.load_global()
that loads 'codebaseGenerator' master configuration parameter. Test
both loading correct parameter and invalid.
unit test error paths in MasterConfig.load_status()

Test that invalid 'status' option in master config file is
handled properly.
unit test error paths in BuilderConfig.__init__()

Test that invalid nextSlave, nextBuild, canStartBuild and
env arguments to BuilderConfig constructor are handled properly.
updated buildbot.config.MasterConfig.check_locks() unit tests

Updated unit tests of buildbot.config.MasterConfig.check_locks()
method to use lock objects wrapped inside LockAccess objects.
Nowadays this is the recommended style of specifying locks in the
documentation.

Add one new unit test to check that using 'bare' lock objects still
works.
use self.patch() to override open() in unit test

Using TestCase.patch() method to override buildin open() in a unit test is
much cleaner.
Merge branch 'config-unit-tests' of git://github.com/elmirjagudin/buildbot
fix typo