I just got the following email from Andrew Robinson (one of our developers) who says that you can already customize error messages if you want to.
The feature that you're requesting here already exists, I implemented it in 6.55 originally...
This modification allows a user to add the line "err_XXXX=<override text>" to their notes.ini file in order to override the text of Notes error 0xXXXX..
The Notes error code for "The server is not responding, please contact your administrator..." is 0x0807. With this enhancement, adding the line "err_0807=This is a test" will make the text "Custom Error 0x0807
: This is a test" appear in the error dialog when a server is down in Notes. This can be used to override the text of any Notes error text of which the hex code is known.
The hex code for "Unable to find path to server..." is 0x0803, so adding the line "err_0803=Custom Text Here" to notes.ini will customize this message.
As of Notes 6.5.5 you CAN customize error messages...