Failing terminals aren't identified in the results of _varnish_terminal_run().

_varnish_terminal_run() returns an array on success, where the array keys are the server ip and port of the terminal.

$result = _varnish_terminal_run('vcl.list');
print_r(array_keys($result));
Array
(
    [0] => 127.0.0.1:6082
)

Successful results are added to this array keyed by the terminal and command:

$ret[$terminal][$command] = $status;

Unsuccessful results are logged to watchdog and added to the results array:

$ret[] = FALSE;

PHP assigns these array keys numerically:

$result = _varnish_terminal_run('vcl.list');
print_r(array_keys($result));
Array
(
    [0] => 127.0.0.1:6082
    [1] => 0
)

This makes it more complex to identify which servers failed.