gather errors from centrality measures

[ebey]

When calculating graph or node-level centrality measures that return an error (e.g. information centrality on the ecoli1 network), save the error in a reactiveValues list and output the list in box at the bottom right of the screen. Link the error message to the place in the app where the error occurred with an error number. Ideally, we will eventually show the user all errors in this way.

Currently, the framework is set up just for the node-level information centrality measure, but the conditionalPanel for the error message box is not working as expected. Fix this first, then work on adding more errors to the list.