add pydra-specific exceptions

[kaczmarj]

Once the API is more stable, I can add specific error objects like NodeError and so on.

[effigies]

An initial shot at an exception hierarchy:

https://github.com/nipy/nipype/pull/2629/files#diff-0a8d2a0cd809b83508e5d17e0e0ceb6f

class NipypeError(Exception):
    pass


class PipelineError(NipypeError):
    pass


class NodeError(PipelineError):
    pass


class WorkflowError(NodeError):
    pass


class MappingError(NodeError):
    pass


class JoinError(NodeError):
    pass


class InterfaceError(NipypeError):
    pass