Exceptions refactoring

Outline of changes to exceptions:

  o Now BstError base class has a `domain` and `reason` member,
    reason members are optional.

  o All derived error classes now specify their `domain`

  o For LoadError, LoadErrorReason defines the error's `reason`

  o Now the scheduler `job` class takes care of sending the error
    reason and domain back to the main process where the last
    exception which caused a child task to fail can be discretely stored.