There is no limitation for the location of error  pages.You can put them where ever in the packages of your web application  .

As I know , you can customize error pages  in your project , not in the application or web servers.[at least in Tomcat]

There is 2 possibilities :
1- There is a prob in the XML file that prevents it from correct behavior.[take a look]
2- The path or name of the error page is not correct.
[/pages/error/fileNotFound.html
for
-------web-inf
|
--------pages
|
----------error
|
fileNotFound.html

** Don't forget : "case sensitivity" of Tomcat
]



I can also send you a sample project,if you want.