Print warning on login page when debug settings are in use

Some config settings are intended for development or debugging purposes,
and should normally not be used on production systems:

• $g_show_detailed_errors != OFF
• $g_display_errors[E_USER_ERROR] != 'halt'
• $g_debug_email !== OFF

We now check for these cases and display a warning on the login page
when these are set incorrectly.

Fixes 0016059