want convenience functions for MultiErrors

[davepacheco]

I've found it useful to have a pair of functions for working with MultiErrors:

• errorForList(list): construct a MultiError from the array of errors list, but if list is empty, returns null, and if list has only one element, returns that error directly. In this way, given any number of errors accumulated during an operation, this function returns an appropriate single error to summarize the result.
• errorForEach(err, func): given a normal error, invokes func(err), but given a MultiError, invokes func(errorN) for each error underlying the MultiError. I've found this useful for operations that accumulate warnings that may or may not be encapsulated with a MultiError.

[davepacheco]

For reviewers interested in seeing these in action:

• errorForList: https://github.com/davepacheco/sdc-manta/blob/dev-RFD-85/lib/alarms/alarms.js#L66-L89
• errorForEach: https://github.com/davepacheco/sdc-manta/blob/dev-RFD-85/cmd/manta-adm.js#L830-L843

[davepacheco]

This change is make prepush clean on SmartOS 32-bit node v0.10.46. I don't expect these changes are OS-dependent, data-model-dependent, or version-dependent.

[paulmelnikow]

I'm using errorForList in icedfrisby. Thanks!

[davepacheco]

Cool!