basic,bus-error: return negative error from errno_from_name

errno_from_name used an unusual return convention where 0 meant
"not found". This tripped up config_parse_syscall_errno(),
which would treat that as success. Return -EINVAL instead,
and adjust bus_error_name_to_errno() for the new convention.

Also remove a goto which was used as a simple if and clean
up surroudning code a bit.