To prevent similar issues from happening in the future, all code to pull
results off of the shards cursors in agg now go through a single function.
This function was enhanced to propagate runtime error codes as we already do
with parse-time errors reported from the shards.
The actual change is in src/mongo/shell/collection.js with the remaining changes
to adjust tests to the new api.
The "ideal" arguments are now an array of pipeline ops and an optional object
with extra top-level options for the command (such as {explain: true}. For
backwards compatibility, we still support the varargs style where each argument
is a pipeline stage, but there is no way to specify extra parameters in this
mode.
