Improve code quality and write tests

node_modules/browserslist/error.js

@@ -0,0 +1,12 @@
+function BrowserslistError (message) {
+  this.name = 'BrowserslistError'
+  this.message = message
+  this.browserslist = true
+  if (Error.captureStackTrace) {
+    Error.captureStackTrace(this, BrowserslistError)
+  }
+}
+
+BrowserslistError.prototype = Error.prototype
+
+module.exports = BrowserslistError