fix: improve error message when virus properties flag is missing

[ivan-aksamentov]

Resolves: #703

Adds missing CLI args check for a situation where --input-dataset is not provided and neither the --input-virus-properties.

Now the error message reads as follows:

[ERROR] Nextclade: Error: Either --input-dataset or --input-virus-properties is required. Cannot proceed without virus properties. Note: the new required file virus_properties.json was introduced in version 1.10.0. You can obtain it from the latest Nextclade dataset. See nextclade dataset get --help and https://docs.nextstrain.org/projects/nextclade for more information

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[corneliusroemer]

Great! This shouldn't break anything so ready for patch release tomorrow once Richard has ok'ed it?