[an error occurred while processing this directive] with custom htaccess protect

So I’ve been trying to secure my WordPress installations by adding some basic WordPress file protection through .htaccess. It works, but the error pages that are blocked are being directed to a page that simply reads:

[an error occurred while processing this directive]

How do I get it to redirect to the 404 page built into theme?

Here is code for my .htaccess file:

# Use PHP5.4 as default
AddHandler application/x-httpd-php54 .php

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

# Disable the Server Signature #
ServerSignature Off

# Deny access to config file #
<files wp-config.php>
order allow,deny
deny from all
</files>

# Prevent directory browsing #
Options All -Indexes

# Protect wp-includes folder and files #
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^wp-admin/includes/ - [F,L]
RewriteRule !^wp-includes/ - [S=3]
RewriteRule ^wp-includes/[^/]+\.php$ - [F,L]
RewriteRule ^wp-includes/js/tinymce/langs/.+\.php - [F,L]
RewriteRule ^wp-includes/theme-compat/ - [F,L]
</IfModule>

# Block Sensitive Files #
Options All -Indexes
<files .htaccess>
Order allow,deny
Deny from all
</files>
<files readme.html>
Order allow,deny
Deny from all
</files>
<files license.txt>
Order allow,deny
Deny from all
</files>
<files install.php>
Order allow,deny
Deny from all
</files>
<files wp-config.php>
Order allow,deny
Deny from all
</files>
<files error_log>
Order allow,deny
Deny from all
</files>