Add a validation message bundle

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Modify keyword validation test to use the message bundle

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Get rid of KeywordValidationMessages enum

All uses replaced with the new bundle.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>