Custom Error Page when servlet is not reachable (tomcat8) - tomcat8

Question

I'm running Tomcat 8.0.21 with java 1.8.0_40
I'm trying to create a costum error page, which should be displayed when I redeploy my application and the servlet is not reachable.
The obvious solution adding:
<error-page>
<error-code>404</error-code>
<location>/ErrorHandler/404.html</location>
</error-page>
to the <tomcatDir>/conf/web.xml won't work here since tomcat always seems to look for the error page in the same servlet. (so e.g. if calling <url>/idonotexist he uses the 404.html located in ROOT/ErrorHandler/404.html and not in ErrorHandler/404.html as I would have expected) If the webapp is down (stopped via the manager app, or due to maintenance) the error page is blank.
If I remove the lines from the <tomcatDir>/conf/web.xml the default tomcat 404 error message is shown though. Is there any possibility to change the default error handling of tomcat?
edit:
ErrorHandler is a deployed webapp with the following web.xml:
<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
version="2.4">
<display-name>Error Application</display-name>
<description>
A webapp for error handling
</description>
<!-- servlet definition -->
<servlet>
<servlet-name>ErrorHandler</servlet-name>
<servlet-class>ErrorHandler</servlet-class>
</servlet>
<!-- servlet mappings -->
<servlet-mapping>
<servlet-name>ErrorHandler</servlet-name>
<url-pattern>/ErrorHandler</url-pattern>
</servlet-mapping>
</web-app>
If I call <tomcatUrl>/ErrorHandler/404.html in my browser I can view the error page just fine.