Use custom toast to expose full error tracebacks to users

[maybeiambatman]

Currently when any action in DART fails, we only show a static toast with the message "Failed :(" and log the error message in the javascript console.

I think we should use Angular's custom toast to expose this message through the UI. We can use the model similar to the custom toast example here: https://material.angularjs.org/latest/demo/toast

Users will be able to click "More Info" to see the full traceback. Thoughts?

[dannymcpherson]

lgtm