get_idn should not throw for virtual instruments

Virtual instruments should either override get_idn to not fail and respond with something useful, or we should rework this so get_idn is not implemented like this for the generic class.

the reason for the failure is that get_idn calls `.ask` which is not implemented for virtual instruments.

An example of this issue is in #4590

Start by tracing get_idn in the generic instrument class and checking how virtual instruments implement or omit .ask. Read the example in issue #4590 to understand the failure. Done means virtual instruments no longer throw from get_idn and provide a useful response or an explicitly supported alternative.

Written by the indexing model from the issue text.