missing variable exception

[litwisha]

Added MissingVariableException exception.
User-defined variables can raise this exception, just in case, if the variable value is missing.
If the exception was raised, than corresponding conditions are negative(false).

Example:
we have an order, and each order has payment attribute, which contains payment country.

class Payment:
    type: str
    country: str
    
class Order:
    payment: Payment = None

we have 3 orders with corresponding payments:

1. Payment(country='DE')
2. Payment(country='AT')
3. none

if we have a condition: payment_country != 'DE', than in this case 2-3 orders will return True, but 3rd order has no payment.

Situations depend on business logic, so MissingVariableException will give more flexibility to user.