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:

gosling.js/src/core/mark/point.ts

Lines 13 to 15 in 9b36b6c

	if (!spec.width || !spec.height) {
	console.warn('Size of a track is not properly determined, so visual mark cannot be rendered');
	return;

gosling.js/src/data-fetchers/vcf/vcf-worker.ts

Lines 183 to 185 in 9b36b6c

	if (Number.isNaN(x) || Number.isNaN(z)) {
	console.warn('Invalid tile zoom or position:', z, x);
	continue;

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.