Keruald sites use GENERAL_ERROR, HACK_ERROR, and SQL_ERROR
to categorize fatal error messages.

One of our current goals is to help sites to eliminate the includes/ directory,
allowing them to refer to Keruald libraries for generic code or to their own
engine classes for domain-specific code.

As such, importing the constants enables the use of the legacy message_die
function, either directly in the global space or migrated to an error handler
class.

Ref T2145

Tested with Obsidian Workspaces code.