Raise an exception when the template cannot be found, and update tests accordingly

lib/vagrant/errors.rb

@@ -784,6 +784,10 @@ module Vagrant
       error_key(:vagrantfile_syntax_error)
     end
 
+    class VagrantfileTemplateNotFoundError < VagrantError
+      error_key(:vagrantfile_template_not_found_error)
+    end
+
     class VagrantfileWriteError < VagrantError
       error_key(:vagrantfile_write_error)
     end

plugins/commands/init/command.rb

@@ -71,6 +71,12 @@ module VagrantPlugins
         # Strip the .erb extension off the template if the user passes it in
         options[:template] = options[:template].chomp(".erb")
 
+        # Make sure the template actually exists
+        full_template_path = Vagrant::Util::TemplateRenderer.new(options[:template], template_root: template_root).full_template_path
+        if !File.file?(full_template_path)
+          raise Vagrant::Errors::VagrantfileTemplateNotFoundError, path: full_template_path
+        end
+
         contents = Vagrant::Util::TemplateRenderer.render(options[:template],
           box_name: argv[0] || "base",
           box_url: argv[1],

templates/locales/en.yml

@@ -1379,6 +1379,9 @@ en:
         message is reproduced below for convenience:
 
         %{file}
+      vagrantfile_template_not_found_error: |-
+        The Vagrantfile template '%{path}' does not exist. Please double check
+        the template path and try again.
       vagrantfile_write_error: |-
         The user that is running Vagrant doesn't have the proper permissions
         to write a Vagrantfile to the specified location. Please ensure that

test/unit/plugins/commands/init/command_test.rb

@@ -61,6 +61,12 @@ describe VagrantPlugins::CommandInit::Command do
       expect(contents).to match(/config.vm.hostname = "vagrant.dev"/)
     end
 
+    it "raises an appropriate exception when the template file can't be found" do
+      expect {
+      described_class.new(["--template", "./a/b/c/template"], env).execute
+      }.to raise_error(Vagrant::Errors::VagrantfileTemplateNotFoundError)
+    end
+
     it "does not overwrite an existing Vagrantfile" do
       # Create an existing Vagrantfile
       File.open(File.join(env.cwd, "Vagrantfile"), "w+") { |f| f.write("") }