Validate the numInitialChunks parameter for 'shardcollection' earlier

ShardCollectionCmd::run relies on Chunk::multiSplit to validate that a chunk is not split into more than 8192 parts as part of a 'shardcollection' command:

    uassert(13333, "can't split a chunk in that many parts", m.size() < maxSplitPoints);

This check should occur earlier to prevent unnecessary work on the mongos process.