SushantRao,
ACS Commons Error Page Handler works fine for POSTs as well. If an error occurs, Sling handles the error and passes it off to the ACS Commons Error Page handle, which in turn internally makes a HTTP GET request for the error page defined by the @errorPages property.
The problem your running into it seems, is you are making POSTs that may "error" (403/500) to paths that dont reside under a hierarchy that that defines a @errorPages, thus the error page to display is ambiguous.
You can set a "System" default for ACS Commons error page handler via OSGi configuration which is described in the ACS Commons Error Page Handler documentation; this can be used to surface error pages for request where a @errorPages cannot be found.
...
Taking a step back, its better practice to bind your POST actions to actual content resources, rather than to arbitrary content endpoints or path-based servlets.
For example:
Good: HTTP POST /content/my-project/actionable-page.some-action.html
Bad: HTTP POST /some-action.html