Fix dbs path, fix db errors

[evlekht]

#60 introduced some bugs with db config and db errors, this PR fixes that.

• Storage should use its "user" package errors, because it depends on its interface. User-package expects certain errors defined by it, because it doesn't know about storage implementation.
• DB pkg should ensure database path, otherwise it would fail if path is missing some folders along the way.
• matrix.storage config field could be removed in favor of general db.path that defines database root directory