Log exceptions with stacktrace from ndb.loglevel

[GoogleCodeExporter]

As of fixing issue 164 the exceptions from tasklets are only logged as 
warnings, without stacktrace. However, if such an exception makes it up to 
ndb.toplevel it most probably won't be captured by anyone else, hence it would 
make sense (imho) to log here the exception again at error level, with 
stacktrace (so that it more prominently stands out in the logs).

Original issue reported on code.google.com by dify....@gmail.com on 27 Aug 2012 at 9:04