Provide a better error message when loading a multi-document yaml

[lengau]

What needs to get done

Right now the workflow when reading a multi-document yaml file returns an internal error:

$ charmcraft pack
charmcraft internal error: ComposerError('expected a single document in the stream', <yaml.error.Mark object at 0x73a1c3d77970>, 'but found another document', <yaml.error.Mark object at 0x73a1c3ce0340>)                                           
Full execution log: '/home/lengau/.local/state/charmcraft/log/charmcraft-20240920-152105.956862.log' 

This should be converted to an error message that we don't support multi-document YAML files.

Why it needs to get done

Strange ux. See: canonical/charmcraft#214

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3411.

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