How to configure error document on webserver(dispatcher)

Use below table to do the work.

Link: http://dev.day.com/docs/en/cq/current/deploying/dispatcher/disp_install.html
Refer to the below table for configurations:

DispatcherConfig	Location and name of the co nfiguration file.
DispatcherLog	Location and name of the log file.
DispatcherLogLevel	Log level for the log file: 0 - Errors 1 - Warnings 2 - Infos 3 - Debug Note: It is recommended to set the log level to 3 during installation and testing, then to 0 when running in a production environment.
DispatcherNoServerHeader	Defines the Server Header to be used: undefined or 0 - the HTTP server header contains the CQ version. 1 - the Apache server header is used.
DispatcherDeclineRoot	Defines whether to decline requests to the root "/": 0 - accept requests to / 1 - requests to / are not handled by the dispatcher; use mod_alias for the correct mapping.
DispatcherUseProcessedURL	Defines whether to use pre-processed URLs for all further processing by Dispatcher: 0 - use the original URL passed to the web server. 1 - the dispatcher uses the URL already processed by the handlers that precede the dispatcher (i.e. mod_rewrite) instead of the original URL passed to the web server. For example, either the original or the processed URL is matched with Dispatcher filters. The URL is also used as the basis for the cache file structure. See the Apache web site documentation for information about mod_rewrite; for example, Apache 2.2. When using mod_rewrite, it is advisable to use the flag 'passthrough|PT' (pass through to next handler) to force the rewrite engine to set the uri field of the internal request_rec structure to the value of the filename field.
DispatcherPassError	Defines how to support 40x error codes for ErrorDocument handling: 0 - the dispatcher spools all error responses to the client. 1 - the dispatcher does not spool an error response to the client (where the status code is greater or equal than 400), but passes the status code to Apache, which e.g. allows an ErrorDocument directive to process such a status code.

Remember:
We need to configure 'DispatcherPassError' and 'DispatcherUseProcessedUrl' parameters.