This displays an error at the top of the screen on page load that
will inform any user with the userAgent string "ZulipDesktop" that
they should upgrade to our newer electron app
Fixes: #6551.
This moves the implementations of error/report/message from
ui.js to ui_report.js. They had been shimmed before, so calling
modules still use the same names to call the functions, but we
no longer need the shims.