Change the error message given?

[Tiger1]

I have placed an expression on a table:

SWITCH(USEREMAIL(),
“gentry.perry@modernusa.com”, “ALL_CHANGES”,
“READ_ONLY”)

This restricts ONLY Gentry to be able to use the form (i created a form). I understand this part - but how can i change the message that pops up - if the user is NOT Gentry? See below: