This commit adds support for structural errors / failures / exceptions
on the elasticsearch REST layer. Exceptions are rendering with at least
a `type` and a `reason` corresponding to the exception name and the message.
Some expcetions like the ones associated with an index or a shard will have
additional information about the index the exception was triggered on or the
shard respectivly.
Each rendered response will also contain a list of root causes which is a list
of distinct shard level errors returned for the request. Root causes are the lowest
level elasticsearch exception found per shard response and are intended to be displayed
to the user to indicate the soruce of the exception.
Shard level response are by-default grouped by their type and reason to reduce the amount
of duplicates retunred. Yet, the same exception retunred from different indices will not be
grouped.
Closes#3303
Simon Willnauer
Martijn van Groningen