Improved Error Messaging for Incorrect URs

[shannona]

We should improve our error messaging for UR types that GST does not recognize, as they are currently terse:

Suggestions:

hdkey: Seed Tool is a storage vault for seeds. Individual keys cannot be imported.

response: Seed Tool does not currently initiate any crypto-requests, so it does not recognize crypto-responses.

These would go after the current (terse) errors.

[shannona]

(We might need extra help text down the line to also indicate what users could do with their hdkey or response, but I don't know that there's a most likely path at the moment.)

[wolfmcnally]

This should be prioritized and placed into a specific milestone.