deps: Add failed optional deps warnings to tree obj

The top level tree holds on to warnings that occured while it was being
generated to be emitted later. This makes it so that optional dependency
failures are treated this way.

Credit: @iarna
PR-URL: #10465
Reviewed By: @othiym23
Fixes: #9204

lib/install/deps.js

@@ -297,6 +297,18 @@ var failedDependency = exports.failedDependency = function (tree, name_pkg) {
   return false
 }
 
+function top (tree) {
+  if (tree.parent) return top(tree.parent)
+  return tree
+}
+
+function treeWarn (tree, what, error) {
+  var topTree = top(tree)
+  if (!topTree.warnings) topTree.warnings = []
+  error.optional = flatNameFromTree(tree) + '/' + what
+  topTree.warnings.push(error)
+}
+
 function andHandleOptionalErrors (log, tree, name, done) {
   validate('OOSF', arguments)
   return function (er, child, childLog) {
@@ -305,8 +317,7 @@ function andHandleOptionalErrors (log, tree, name, done) {
     var isFatal = failedDependency(tree, name)
     if (er && !isFatal) {
       tree.children = tree.children.filter(noModuleNameMatches(name))
-      log.warn('install', "Couldn't install optional dependency:", er.message)
-      log.verbose('install', er.stack)
+      treeWarn(tree, name, er)
       return done()
     } else {
       return done(er, child, childLog)