Provide custom error when attempting to push too many files with FTP

When the configured directory for FTP push has too many files, it will
generate an exception and fail due to a stack overflow. When this happens
just rescue out the exception and re-raise a custom error to provide
some context to the user on the actual problem.

plugins/pushes/ftp/errors.rb

@@ -0,0 +1,13 @@
+module VagrantPlugins
+  module FTPPush
+    module Errors
+      class Error < Vagrant::Errors::VagrantError
+        error_namespace("ftp_push.errors")
+      end
+
+      class TooManyFiles < Error
+        error_key(:too_many_files)
+      end
+    end
+  end
+end

plugins/pushes/ftp/locales/en.yml

@@ -9,3 +9,8 @@ en:
             config.push.define "ftp" do |push|
               push.%{attribute} = "..."
             end
+      too_many_files: |-
+        The configured directory for Vagrant FTP push contains too many files
+        to successfully complete the command. This can be resolved by either
+        removing extraneous files from the configured directory, or updating
+        the `dir` configuration option to a subdirectory.

plugins/pushes/ftp/push.rb

@@ -2,6 +2,7 @@ require "net/ftp"
 require "pathname"
 
 require_relative "adapter"
+require_relative "errors"
 
 module VagrantPlugins
   module FTPPush
@@ -17,12 +18,17 @@ module VagrantPlugins
       def push
         # Grab files early so if there's an exception or issue, we don't have to
         # wait and close the (S)FTP connection as well
+        files = nil
+        begin
           files = Hash[*all_files.flat_map do |file|
               relative_path = relative_path_for(file, config.dir)
               destination = File.join(config.destination, relative_path)
               file = File.expand_path(file, env.root_path)
               [file, destination]
             end]
+        rescue SystemStackError
+          raise Errors::TooManyFiles
+        end
 
         ftp = "#{config.username}@#{config.host}:#{config.destination}"
         env.ui.info "Uploading #{env.root_path} to #{ftp}"

test/unit/plugins/pushes/ftp/push_test.rb

@@ -73,6 +73,11 @@ describe VagrantPlugins::FTPPush::Push do
       subject.push
       expect(server.files).to eq(%w(Gemfile data.txt))
     end
+
+    it "raises informative exception when too many files to process" do
+      expect(subject).to receive(:all_files).and_raise(SystemStackError)
+      expect{ subject.push }.to raise_error(VagrantPlugins::FTPPush::Errors::TooManyFiles)
+    end
   end
 
   describe "#connect" do