Implement user-defined error handler mechanism

[jpsamaroo]

We could provide an option to let users define an error handler function which will handle all kinds of task failures (which may necessitate propagating more richly-annotated information on error source/reason). If this could be defined as a regular scheduler option (e.g. settable with with_options), it might alleviate questions about retries for failed operations.

This should also not be specific to errors from user task code - this should generalize to errors coming from basically anywhere in Dagger, including the storage system in #289 , the fault handler, etc.