Emit DownsampleFailed when the output size is still too large

[mattrunyon]

We currently enable optional downsampling over 30k points and mandate it over 200k (for time series line plots). This is to keep the web UI responsive and reduce unnecessary data transmission.

There is an issue when downsampling a table with nulls. Nulls are preserved so that gaps appear in the plot where necessary; however, this can lead to downsampling resulting in over 200k points.

The problem is at a certain table size w/ certain frequency of nulls, the downsample call still returns hundreds of thousands of points. The UI struggles to render these. If you had a table with alternating null and non-null values, downsampling would effectively do nothing, but we would still try to plot it.

It would be nice to have some way of limiting the output size of downsampling from the client and emitting a failure or some other message to the client when it is exceeded. We could check these on the web side, but that still results in a large data transfer to the web which we would then just ignore if plots don't render in a timely manner at that size.

Related #4563 when you reach a certain number of nulls, you also exceed the 100MB payload size limit and just get an infinite spinner.