Improve platform validation error message

[cmatsuoka]

What needs to get done

Display better validation error messages to the user when platforms are not correctly defined. This is a follow-up to #

Why it needs to get done

This is the current validation error message:

wheelcraft internal error: ValidationError(model='BuildPlanner', errors=[{'loc': ('platforms', 'build-on'), 'msg': 'value is not a valid list', 'type': 'type_error.list'}])  

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-2919.

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