Fixed how errors were displayed when variant limits were reached. Fix…

[joshua-davis]

…es fogbugz 1355815

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Purpose of this PR

https://fogbugz.unity3d.com/f/cases/1355815/

The behavior of this bug has shifted over the past few months. The last main fix that caused the latest behavior was #3664. This removed a validation error being added to each preview node which is how the old error message used to be displayed. This caused each node to now fall back to the shader compiler error. This shader compiler error was because we put both the error shader and the original shader in the same preview, something akin to:

Shader "MyShaderGraph" {...}
Shader "hidden/preview/error_eff8e5151a46414e929efe020074aa57" {}

Removing the duplicate shaders removed the unreadable error, but left the user with only a log message and the pink preview shaders.

To aid discoverability, a help box was added to the blackboard when the variant limit was reached.

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Testing status

• Blackboard error appears when adding enough keywords to exceed the limit
• Error disappears when removing a keyword
• Error disappears when undoing (to get back below the variant limit)
• Error re-appears when redoing back over the variant limit
• Nodes with previews now no longer display an error badge when over the variant limit.
  • They still display as an error (pink shader) which was deemed fine.
• Console log message still printed with the variant exceeded error
• Tested the above in both a sub-graph and a graph

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[joshua-davis]

This prevents the duplicate shader from being put in the same file.

[jessebarker]

LGTM

[Nightmask3]

code looks good! approving 👍

[bencloward]

Tested this out locally and it's working well. Very clear message now.