Improve startup error messages for corrupted state

[msm-code]

The startup script creates a database if it doesn't exist:

#!/bin/bash

cd /var/lib/ursadb

if [ ! -f "$1" ]
then
    /usr/bin/ursadb_new "$1"
fi

/usr/bin/dumb-init -- /usr/bin/ursadb $@

This checks if a file named $1 already exists. Notably, it doesn't hande the case when $1 exists but is a directory. This situation is also not handled well by the C++ code.

We should improve error message in this situation and guide users somewhat.