error message for moveChunk is confusing with incorrect shard key specified

If the moveChunk "find" document does not contain a correct shard key, it may return:

assert: command failed: {

	"code" : 13141,

	"ok" : 0,

	"errmsg" : "exception: Chunk map pointed to incorrect chunk"

} : undefined

...which is pretty cryptic. This is a pretty easy failure to detect and one that should be reported more clearly.