test: add lib/utils/error-handler.js tests

Add unit tests to `lib/utils/error-handler.js`, these are very special
since the module handles some internal state through variables which are
not exposed and binds itself to multiple global `process` events.

Also two minor tweaks/fixes to the original implementation:
- Refactored unused param in `reallyExit()`
- Fixed String.prototype.match group capture usage

PR-URL: #1742
Credit: @ruyadorno
Close: #1742
Reviewed-by: @isaacs

lib/utils/error-handler.js

@@ -90,9 +90,7 @@ const exit = (code, noLog) => {
   log.verbose('exit', code)
   if (log.level === 'silent') noLog = true
 
-  const reallyExit = (er) => {
-    if (er && !code) code = typeof er.errno === 'number' ? er.errno : 1
-
+  const reallyExit = () => {
     itWorked = !code
 
     // Exit directly -- nothing in the CLI should still be running in the
@@ -141,9 +139,9 @@ const errorHandler = (er) => {
     return exit(1, true)
   }
 
-  const m = er.code || er.message.match(/^(?:Error: )?(E[A-Z]+)/)
-  if (m && !er.code) {
-    er.code = m
+  if (!er.code) {
+    const matchErrorCode = er.message.match(/^(?:Error: )?(E[A-Z]+)/)
+    er.code = matchErrorCode && matchErrorCode[1]
   }
 
   for (const k of ['type', 'stack', 'statusCode', 'pkgid']) {

tap-snapshots/test-lib-utils-error-handler.js-TAP.test.js

@@ -0,0 +1,23 @@
+/* IMPORTANT
+ * This snapshot file is auto-generated, but designed for humans.
+ * It should be checked into source control and tracked carefully.
+ * Re-generate by setting TAP_SNAPSHOT=1 and running tests.
+ * Make sure to inspect the output below.  Do not ignore changes!
+ */
+'use strict'
+exports[`test/lib/utils/error-handler.js TAP handles unknown error > should have expected log contents for unknown error 1`] = `
+0 verbose code 1
+1 error foo A complete log of this run can be found in:
+1 error foo     {CWD}/cachefolder/_logs/expecteddate-debug.log
+2 verbose stack Error: ERROR
+3 verbose cwd {CWD}
+4 verbose Foo 1.0.0
+5 verbose argv "/node" "{CWD}/test/lib/utils/error-handler.js"
+6 verbose node v1.0.0
+7 verbose npm  v1.0.0
+8 error foo code ERROR
+9 error foo ERR ERROR
+10 error foo ERR ERROR
+11 verbose exit 1
+
+`