Update loggers.py

If the active log file (at least stdout-log for a supervised proccess) is removed, the log file will not be recreated. When loggers.py tries to do "os.rename(self.baseFilename, dfn)" in RotatingFileHandler.removeAndRename (called by RotatingFileHandler.doRollover) it will fail with OSError, no new file is opened, and the managed process will later be stuck in a write.

It's not uncommon with cleanup scripts in /tmp, and /tmp is the default location for the logs...
added test case for current log removed in RotatingFileHandlerTests
Merge pull request #402 from magnus-lycka/master

Log rotation fails if current log was removed. Log lost until restart.