If an SQLite database exists, but is not writable, return a lowLevelError

Also, the check for database directory existence and writablity can be accomplished with is_writable which returns TRUE if present AND writable. Addresses issue #141.

Signed-off-by: Max Fierke <max@maxfierke.com>
Merge pull request #142 from maxfierke/master

If an SQLite database exists, but is not writable, return a lowLevelError