Add EADDRINUSE test

[cgewecke]

This error appears to be uncatch-able. It throws...but I can't even catch it in a test try block. Idk.

Adding a skipped test, some comments & cleaning up the UI message previously written for this.

[codecov-io]

Codecov Report

Merging #404 into truffle-plugin will not change coverage.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           truffle-plugin     #404   +/-   ##
===============================================
  Coverage           96.82%   96.82%           
===============================================
  Files                  12       12           
  Lines                 629      629           
===============================================
  Hits                  609      609           
  Misses                 20       20

Impacted Files	Coverage Δ
lib/ui.js	100% <ø> (ø)	⬆️
lib/app.js	96.22% <100%> (ø)	⬆️

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