Better indication of errors in the state graph

[GoogleCodeExporter]

Currently, in the state graph visualization in the exploration editor, there 
are two issues:

1) If you're on a state that has an error, you cannot tell that it has an error.
2) States that have errors are colored red, but this isn't necessarily 
informative.

Some potential fixes are:

1) If you're on a state that has an error, color the node red instead of 
coloring it blue
2) Instead of using colors for errors, implement a system where states have 
little warning signs attached to them (as a bonus, if you hover over a warning 
sign, you should get a list of errors for that state)

Original issue reported on code.google.com by amitdeut...@google.com on 10 Apr 2015 at 5:54