fix(tap): report an unreadable database instead of listing no taps

[indaco]

Description

mt tap and mt --json tap no longer report a broken database as an empty tap list.

When malt.db exists but cannot be opened (unreadable, corrupt, or db/ not accessible), both commands now print "Cannot open the tap database" on stderr, leave stdout empty, and exit 1. Before, they printed "No taps registered" / [] and exited 0. Scripts and --json consumers could not tell a broken database from a real empty list.

Only an absent db/ still reads as an empty tap list. A db/ that exists without a malt.db and cannot be written now fails the same way, matching mt list.

Related Issue

• None

Notes for Reviewers

• The mode-000 checks skip when run as root.