Improve panic error message to include the parse error.

[brianpursley]

This came up while investigating issue #4988.

If my openapi json file fails to parse (due to a missing curly brace), the error message provides only this error message:

panic: invalid schema file

This PR adds the error message to the panic.

The new message will provide more details about the problem:

panic: invalid schema file: json: cannot unmarshal Go value of type spec.Swagger: unexpected EOF

[brianpursley]

/assign @natasha41575

[KnVerey]

Ideally we would not panic, but I agree that this is a worthwhile improvement in the interim.

/lgtm
/approve

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[brianpursley]

Ideally we would not panic, but I agree that this is a worthwhile improvement in the interim.

@KnVerey agreed, and I was going to attempt that, but there were going to be too many cascading effects. I was surprised to see panic used so routinely here and other places.