xapian-db: improve some error messages

Correctly handle re-opening a transaction after one has finished.

Recognize some database opening errors and give some better user hints.

Fixes #2615.

lib/mu-xapian-db.cc

@@ -98,7 +98,7 @@ make_db(const std::string& db_path, Flavor flavor)
 	}
 }
 
-XapianDb::XapianDb(const std::string& db_path, Flavor flavor) :
+XapianDb::XapianDb(const std::string& db_path, Flavor flavor):
 	path_(make_path(db_path, flavor)),
 	db_(make_db(path_, flavor)),
 	batch_size_{Config(*this).get<Config::Id::BatchSize>()}