add helper to check if path is below another

this is used in the Database manager
raise exception when adding msg no non-indexable path

i.e., one not below that of notmuch's root path
fix: make sure DBManager is initialized correctly...

and knows the path to notmuchs root dir
move check to add_message directly
adjust error message
handle errors caused by trying to index mails...

... not below notmuchs root path

issue #611
Merge branch '0.3.4-fix-611'