assert-pre-internal.h: append "error is" to first message

The previous message, "Library precondition not satisfied", was
misleading because the following message does not indicate the
precondition, but how the precondition was not satisfied.

Making it "Library precondition not satisfied; error is:" here to make
it clear that the following message indicates what the error is.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>