kernel/v2: capture error if provider config is bad [GH-2959]

plugins/kernel_v2/config/vm.rb

@@ -371,8 +371,14 @@ module VagrantPlugins
           # Load it up
           config    = config_class.new
 
-          blocks.each do |b|
-            b.call(config, Vagrant::Config::V2::DummyConfig.new)
+          begin
+            blocks.each do |b|
+              b.call(config, Vagrant::Config::V2::DummyConfig.new)
+            end
+          rescue Exception => e
+            raise Vagrant::Errors::VagrantfileLoadError,
+              path: "<provider config: #{name}>",
+              message: e.message
           end
 
           config.finalize!

test/unit/plugins/kernel_v2/config/vm_test.rb

@@ -114,6 +114,34 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
     end
   end
 
+  describe "#provider and #get_provider_config" do
+    it "compiles the configurations for a provider" do
+      subject.provider "virtualbox" do |vb|
+        vb.gui = true
+      end
+
+      subject.provider "virtualbox" do |vb|
+        vb.name = "foo"
+      end
+
+      subject.finalize!
+
+      config = subject.get_provider_config(:virtualbox)
+      expect(config.name).to eq("foo")
+      expect(config.gui).to be_true
+    end
+
+    it "raises an exception if there is a problem loading" do
+      subject.provider "virtualbox" do |vb|
+        # Purposeful bad variable
+        vm.foo = "bar"
+      end
+
+      expect { subject.finalize! }.
+        to raise_error(Vagrant::Errors::VagrantfileLoadError)
+    end
+  end
+
   describe "#provision" do
     it "stores the provisioners" do
       subject.provision("shell", inline: "foo")