Do not panic -- it's only a user error

[webbnh]

Describe the bug

Per #85 (comment), we should not be issuing a panic as a result of a user error. The error should be detected at an earlier point when it is reasonable to return an error to the user. A panic, aside from being ugly, is likely to be misconstrued by a user as a problem internal to Arca. We should instead be reporting an error as we would in other cases of a malformed workflow file.

[jaredoconnell]

For this task, we will need to add a recursive validation function to report errors before they're used.