eliminate guesswork with Vagrantfile errors

If the Vagrantfile has some kind of error, display not only
its path and the exception message, but also the originating
line number and exception class.

Also log the full backtrace when the error is in a provider
block, just as it is done when it's outside a provider block.

lib/vagrant/config/loader.rb

@@ -212,6 +212,8 @@ module Vagrant
             # Report the generic exception
             raise Errors::VagrantfileLoadError,
               path: path,
+              line: e.backtrace[0].split(':')[1],
+              exception_class: e.class,
               message: e.message
           end
         end

plugins/kernel_v2/config/vm.rb

@@ -37,6 +37,8 @@ module VagrantPlugins
       attr_reader :provisioners
 
       def initialize
+        @logger = Log4r::Logger.new("vagrant::config::vm")
+
         @base_mac                      = UNSET_VALUE
         @boot_timeout                  = UNSET_VALUE
         @box                           = UNSET_VALUE
@@ -446,8 +448,13 @@ module VagrantPlugins
               config = config.merge(new_config)
             end
           rescue Exception => e
+            @logger.error("Vagrantfile load error: #{e.message}")
+            @logger.error(e.backtrace.join("\n"))
+
             raise Vagrant::Errors::VagrantfileLoadError,
               path: "<provider config: #{name}>",
+              line: e.backtrace[0].split(':')[1],
+              exception_class: e.class,
               message: e.message
           end
 

templates/locales/en.yml

@@ -1200,7 +1200,8 @@ en:
         a syntax error.
 
         Path: %{path}
-        Message: %{message}
+        Line number: %{line}
+        Message: %{exception_class}: %{message}
       vagrantfile_syntax_error: |-
         There is a syntax error in the following Vagrantfile. The syntax error
         message is reproduced below for convenience: