Convert console.warn() messages to Errors

[etowahadams]

Currently, there are many places within Gosling where when an error occurs, it is just logged to the console.

For example: https://github.com/gosling-lang/gosling.js/blob/9b36b6c473c9707907c4cc86a8df7a07b150b1be/src/core/mark/point.ts#L13-L15

https://github.com/gosling-lang/gosling.js/blob/9b36b6c473c9707907c4cc86a8df7a07b150b1be/src/data-fetchers/vcf/vcf-worker.ts#L183-L185

Some of these may be appropriate to leave as console warnings, but probably many of these should throw some Error instead, which can be caught at the top level so that other applications that use Gosling can more easily access them.