Polish mangle error messages

[boopathi]

Ref #547

[codecov]

Codecov Report

Merging #548 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #548   +/-   ##
=======================================
  Coverage   82.89%   82.89%           
=======================================
  Files          40       40           
  Lines        2725     2725           
  Branches      956      956           
=======================================
  Hits         2259     2259           
  Misses        281      281           
  Partials      185      185

Impacted Files	Coverage Δ
...ages/babel-plugin-minify-mangle-names/src/index.js	81.96% <ø> (ø)	⬆️
...el-plugin-minify-mangle-names/src/scope-tracker.js	71.01% <ø> (ø)	⬆️

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