Be more defensive when trying to get the line number

Since this is the last line of defense before raising an error, we want to make
sure we don't cause an error while trying to render the error.

lib/vagrant/config/loader.rb

@@ -209,10 +209,15 @@ module Vagrant
             @logger.error("Vagrantfile load error: #{e.message}")
             @logger.error(e.backtrace.join("\n"))
 
+            line = "(unknown)"
+            if e.backtrace && e.backtrace[0]
+              line = e.backtrace[0].split(":")[1]
+            end
+
             # Report the generic exception
             raise Errors::VagrantfileLoadError,
               path: path,
-              line: e.backtrace[0].split(':')[1],
+              line: line,
               exception_class: e.class,
               message: e.message
           end

plugins/kernel_v2/config/vm.rb

@@ -449,11 +449,18 @@ module VagrantPlugins
             end
           rescue Exception => e
             @logger.error("Vagrantfile load error: #{e.message}")
+            @logger.error(e.inspect)
+            @logger.error(e.message)
             @logger.error(e.backtrace.join("\n"))
 
+            line = "(unknown)"
+            if e.backtrace && e.backtrace[0]
+              line = e.backtrace[0].split(":")[1]
+            end
+
             raise Vagrant::Errors::VagrantfileLoadError,
               path: "<provider config: #{name}>",
-              line: e.backtrace[0].split(':')[1],
+              line: line,
               exception_class: e.class,
               message: e.message
           end