Route all errors through reporter

[cgewecke]

PR:

• Re-organizes and cleans up the truffle plugin
• Makes UI an extensible class. Separates plugin / app error reporting
• Adds error handling for .solcover.js load failures (syntex error, etc)
• Adds cli flags to force use globally installed truffle and plugin supplied truffle lib
• Adds fallback truffle lib bundle map file (was throwing a transformation error)
• Routes all errors through the UI class and adds tests for:
  • lib location forcing flags
  • instrumentation errors
  • lib module load errors
  • missing sources folder errors
  • solcover loading errors

[codecov-io]

Codecov Report

Merging #399 into truffle-plugin will increase coverage by 3.36%.
The diff coverage is 95.6%.

@@                Coverage Diff                 @@
##           truffle-plugin     #399      +/-   ##
==================================================
+ Coverage           92.61%   95.98%   +3.36%     
==================================================
  Files                  10       11       +1     
  Lines                 542      573      +31     
==================================================
+ Hits                  502      550      +48     
+ Misses                 40       23      -17

Impacted Files	Coverage Δ
dist/plugin-assets/truffle.ui.js	100% <100%> (ø)
dist/truffle.plugin.js	100% <100%> (+14.1%)	⬆️
lib/ui.js	100% <100%> (+18.18%)	⬆️
lib/app.js	93.91% <87.87%> (+2.82%)	⬆️

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