dispatch: split global error handling out so it can be reused We may want a similar error handling at worker.py. This patch extracts the error handling logic to "callcatch" so it can be reused.

py3: conditionalize _winreg import _winreg module is renamed to winreg in python 3. Added the conditionalize statements in the respective file because adding this in pycompat will result in pycompat throwing error as this is a windows registry module and we have buildbots and most of the contributors on linux.