Added + to connect error message

[Pchatain]

Before submitting

• [Typo] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [Yes] Did you read the contributor guideline?
• [No] Did you make sure to update the docs?
• [No] Did you write any new necessary tests?

What does this PR do?

Fixes the error message for when a custom model is not recognized by the model registry. Previously, only the first string was printed, f"Could not infer model type from {cfg}.", and not the later strings which are useful for debugging. Adding the connector + allows all three strings to be printed together following a failed assertion.

PR review

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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