Improve Error Messaging when Yarn is missing

Prior to this commit, the `yarn` command being inaccessible resulted in
confusing failures messages.

When an application depends on `yarn` (by being configured with
`yarn: true`) and the `yarn` command is not acccessible, an
`EmberCli::DependencyError` will be raised.

Author: drcapulet

lib/ember_cli/path_set.rb

@@ -92,7 +92,16 @@ def npm
 
     def yarn
       if yarn?
-        @yarn ||= path_for_executable("yarn")
+        @yarn ||= path_for_executable("yarn").tap do |yarn|
+          unless File.executable?(yarn.to_s)
+            fail DependencyError.new(<<-MSG.strip_heredoc)
+                EmberCLI has been configured to install NodeJS dependencies with Yarn, but the Yarn executable is unavailable.
+
+                Install it by following the instructions at https://yarnpkg.com/lang/en/docs/install/
+
+            MSG
+          end
+        end
       end
     end
 

lib/ember_cli/shell.rb

@@ -41,7 +41,7 @@ def install
         clean_ember_dependencies!
       end
 
-      if paths.yarn.present? && Pathname.new(paths.yarn).executable?
+      if paths.yarn
         run! "#{paths.yarn} install"
       else
         run! "#{paths.npm} prune && #{paths.npm} install"

spec/lib/ember_cli/path_set_spec.rb

@@ -212,13 +212,25 @@
     end
 
     context "when the executable isn't installed on the system" do
-      it "returns nil" do
-        stub_which(yarn: nil)
-        path_set = build_path_set
+      context "and yarn is requested" do
+        it "raises a DependencyError" do
+          stub_which(yarn: nil)
+          app = build_app(options: { yarn: true })
+          path_set = build_path_set(app: app)
 
-        yarn = path_set.yarn
+          expect { path_set.yarn }.to raise_error(EmberCli::DependencyError)
+        end
+      end
+
+      context "and yarn is not requested" do
+        it "returns nil" do
+          stub_which(yarn: nil)
+          path_set = build_path_set
+
+          yarn = path_set.yarn
 
-        expect(yarn).to be_nil
+          expect(yarn).to be_nil
+        end
       end
     end
   end