Read/Write abortion on "Unresolvable Siblings" error

[ajtack]

"If debugging is the process of removing bugs, then programming must be the process of putting them in."

Currently, failure to resolve siblings (by throwing an exception, for instance) will cause undefined behavior and leaking. Users need to be able to abort a read or write if they encounter a sibling set that they cannot resolve. Their server can then report an error that allows them to resolve the case.

Suggest:

1. A new error code riak::errc::unresolvable_siblings to report as the reason for the failed read/write to the application.
2. An exception type (maybe riak::unresolvable_siblings? Check how to interact with system_error.) that sibling resolution routines can throw when they get into trouble.

[ajtack]

4190c4b Partially addresses this, at least making the behavior defined.