Ensure bower executable exists on $PATH

Adds coverage for missing `bower` executable.

To deal with a missing `bower` executable, ensure the `path` is not
`nil` before turning into a `Pathname`.

lib/ember_cli/path_set.rb

@@ -65,7 +65,7 @@ def bower
         bower_path = app_options.fetch(:bower_path) { which("bower") }
 
         bower_path.tap do |path|
-          unless Pathname(path).executable?
+          unless Pathname(path.to_s).executable?
             fail DependencyError.new <<-MSG.strip_heredoc
             Bower is required by EmberCLI
 

spec/lib/ember_cli/path_set_spec.rb

@@ -114,6 +114,17 @@
 
       expect(path_set.bower).to eq fake_bower
     end
+
+    context "when it is missing from the $PATH" do
+      it "raises a helpful exception" do
+        stub_which(bower: nil)
+
+        path_set = build_path_set
+
+        expect { path_set.bower }.
+          to raise_error(EmberCli::DependencyError, /bower is required/i)
+      end
+    end
   end
 
   describe "#npm" do