Apache Httpd Custom ErrorDocument 404 When ProxyPass returned 404
You can avoid proxying for a specific directory by specifying a ! in place of the proxy target. Since it acts on a directory, move error.html into a subdirectory (we'll say errors), and:
<VirtualHost *:80>
ServerName servername
DocumentRoot /somepath/
ProxyPass /errors !
ProxyPass / http://localhost:8080/someapp/
ProxyPassReverse / http://localhost:8080/someapp/
ProxyErrorOverride On
ErrorDocument 404 /errors/error.html
</VirtualHost>