customError on web.config-originated Server Error - asp.net

Question

NOTE
I've determined the the problem I'm describing below is specific to an error enountered loading DLL files specified in the web.config file. I would like to present a user-friendly error even in the case of web.config errors.
END NOTE
When my ASP.Net application encounters a server error I would like it to present a custom error message to the user instead of the following default scary message.
Server Error in '/' Application.
Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.
Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="Off"/>
</system.web>
</configuration>
Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>
I wrote a very simple HTML page and put it in the root of my application. It is called MaintenancePage.htm.
I've set my web.config file to the following:
<customErrors mode="RemoteOnly" defaultRedirect="MaintenancePage.htm">
<error statusCode="404" redirect="PageNotFound.aspx" />
</customErrors>
I've also tried ~/MaintenancePage.htm and http://[mysite]/MaintenancePage.htm. None of those options seem to work.
The way I'm testing this is to rename a DLL my project depends upon and then loading up the site in a web browser. I would expect that since there is an error and a defaultRedirect set there should be no problem showing the error page, however, I'm obviously wrong.
I've search around concerning this problem and it seem that most people are trying to redirect to an aspx page, and are encountering error with doing that. Many even report that they can't get the aspx page to load as a defaultRedirect, but they can get an html page to load.
What could I be doing wrong here?
I should note that I'm doing my testing from a different network outside the company firewall, so changing RemoteOnly to On is not the issue per the documentation. In testing changing RemoteOnly to On had no effect, as expected.

Answer 1

Change RemoteOnly to On.
RemoteOnly:
Specifies that custom errors are shown only to remote clients and
ASP.NET errors are shown to the local host
Also, your URL may be absolute or relative.
Read more about the customErrors settings
To watch it work locally:
<customErrors mode="On" defaultRedirect="/MaintenancePage.htm">
<error statusCode="404" redirect="/PageNotFound.aspx" />
</customErrors>
Easier way to test.
Try a bad url and watch your 404 error take over
Put this on your default.aspx page <% throw new Exception("gaah"); %>
And one other thing to consider: Error Logging. Can be achieved easily through ELMAH.