Manticore UI: Indicate states that terminated in an error

[langston-barrett]

We should add a visual indicator that a state ended with an error, the same way we indicate states that contain potential bugs. I'm thinking either adding text like State N: exploration error in the state banner, or colouring the banner in orange/red. Or even adding some picture like a red exclamation mark at the beginning of the state banner...

Reminder: a state that ends with an error means that the error_msg field of the exploration node will be set.