Graceful image and document error handling

[nickmoreton]

Code in the pull request is AI generated

This pull request introduces improved error handling for image and document downloads from Bynder.

Error handling improvements:

• Added new exception classes
• Updated the download_file function in utils.py to catch HTTP/network errors, check for error pages when expecting images, and raise the appropriate new exceptions.

Model and asset update logic:

• Modified update_file in models.py to catch download-related exceptions and log warnings, skipping updates if errors occur and ensuring no stale state is left on the asset instance.

Testing and validation:

• Expanded tests in test_models.py to verify graceful handling of download errors, file-too-large errors, and invalid content, ensuring asset state remains consistent and no exceptions are propagated.

Dependency update:

• Updated bynder-sdk dependency to allow newer minor versions (<2.1).

[nickmoreton]

I'm happy for this to be closed in favour of #43