fix: Add a try_create method on Reader

[tmathern]

Changes in this pull request

Add Reader factory

Adds a try_create method on Reader class (as class method) that acts as a factory method for Reader objects:

• if there is JUMBF (c2pa/manifest) data to read, from_asset will return a Reader instance
• if there is no JUMBF (c2pa/manifest) data to read, returns None instead of throwing

Fix exception classes hierarchy

• Fixes exception hierarchies to be able to leverage typed exceptions better
• Still keeps backwards compatibility with previous exception handling

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.