Make "null values" visualization warning less obtrusive

[cjcenizal]

CC @elastic/kibana-design @elastic/kibana-visualizations

From https://discuss.elastic.co/t/kibana-keeps-displaying-notice-about-area-charts-and-null-values-even-after-i-change-the-chart-type/107094/3:

This [warning] happened after I added some new fields to my index and started populating them with data. If I'm graphing a timeframe that includes the time before those fields were added then they are in fact null and thus the message appears. If I change the timeframe to only include the time after I started populating those new fields then the message goes away.

This is a suboptimal UX because the message is misleading and the warning interferes with the visualization. I think we can address this with a couple changes:

• We should probably remove the sentence, "A line chart is recommended when displaying data with null values," since it's confusing if the user is already using a line chart.
• We should also probably hide the message itself and show an unobtrusive "warning" icon instead. Clicking the icon should display the message and allow you to dismiss both the message and the icon. @snide What do you think?

@elastic/kibana-sharing I'm CC'ing you too because this will have an impact on Dashboard panels.

[willemdh]

It would be much nicer if just a very small clickable warning icon would be shown instead of this huge orange warning...

In the above case I want to show the cumulative iowait, but this warning makes this graph unusable..

What makes it even worse, is that when change this into a line graph, the warning persists...

Grtz

[snide]

These were the designs we had for this stuff originally. I agree that it shouldn't show inline unless the entire chart can't render.

[cchaos]

If someone can help me get null values into the sample data, I can take a look into this one.

[ghudgins]

We no longer display errors in this format. Resolving this issue as we now show errors in toast messages