SERVER-12708 Report errors from shards when mergingPresorted results.

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.
Revert "add mongoadmin to gitignore"

This reverts commit 359f4cbfeacd9a952fb785ad469cad06eb6e8fcc.
Revert "SERVER-8391 mongoadmin initial commit"

This reverts commit 0f7c3ffaf78b241c0a491232e993ee7d695d02cd.
Merge remote-tracking branch 'upstream/master'