Set unsubscribe() to throw on no such subscription

So far, `flight_vars::unsubscribe()` establishes nothing is done in case
of given subscription doesn't exists. As exposed in issue #34, that goes
against the protocol architecture. And it's a bad practice as well.

This change renames `unknown_variable_error` and
`unknown_subscription_error` as `no_such_variable_error` and
`no_such_subscription_error` respectively. This is due to harmonize with
other exceptions describing missing data.

In addition, it redefines the signature of `unsubscribe()` indicating a
throws clause for `no_such_subscription_error`.

Finally, some wrong throws clause have been fixed.