Improve error message: "Database access parameter 'type' does not specify a supported database backend:mysql"

name: Improve error message: "Database access parameter 'type' does not specify a supported database backend:mysql"
about: Please make it clearer why this error is being emitted.  The 'type' is a valid configuration option.  The problem is that the build does not include support for mysql back-end.

These error messages don't explain well what the problem is and where to look to fix it:

root@debian:/opt/kea-1.5.0# 2019-02-18 17:26:12.746 ERROR [kea-dhcp4.dhcp4/51240] DHCP4_CONFIG_LOAD_FAIL configuration error using file: /usr/local/etc/kea/kea-dhcp4.conf, reason: Unable to open database: Database access parameter 'type' does not specify a supported database backend:mysql

2019-02-18 17:26:12.746 ERROR [kea-dhcp4.dhcp4/51240] DHCP4_INIT_FAIL failed to initialize Kea server: configuration error using file '/usr/local/etc/kea/kea-dhcp4.conf': Unable to open database: Database access parameter 'type' does not specify a supported database backend:mysql

A better message would be something like:

"Unable to open database: The Kea server has not been built with support for database type: mysql"

See #14213