Use 'panic', not 'error'

[langston-barrett]

There are many places in the codebase where we use error, but panic would be more appropriate. These instances usually represent something the implementor believes is a bug or would be impossible, and panic (1) tells users that it's not their fault and (2) provides users with a suggestion of what to do: file a bug.

I would suggest adding the following to our .hlint.yaml, so we discourage this pattern in the future:

- error:
    lhs: "error x"
    rhs: 'panic "nameOfFunction" [x, "more lines of details"]'