This patch adds icons to the right side of nodes when they encounter a
a problem. When hovered, a tooltip displays describing the encountered
while evaluating the node.
Some examples are: attribute doesn't exist, mesh has no faces,
incorrect attribute type, etc. Exposing more messages to the system
will be an ongoing process. Multiple warnings per node are supported.
The system is implemented somewhat generically so that the basic
structure can also be used to store more information from evaluation
for the interface, like a list of available attributes.
Currently the messages are just button tooltips. They could be styled
differently in the future. Another limitation is that every instance of
a node group in a parent node tree will have the same error messages,
the "evaluation context" used to decide when to display the tooltips
must be extended to support node tree paths.
Differential Revision: https://developer.blender.org/D10290
