improve error handling

src/alpm_utils.vala

src/common_daemon.vala

src/daemon.vala

src/daemon_interface.vala

src/error.vala

src/meson.build

src/transaction-cli.vala

src/transaction.vala

src/transaction_interface.vala

src/transaction_interface_daemon.vala

src/transaction_interface_root.vala