Handle package not found error when opening GitHub URLs

[tommy-mitchell]

When opening a non-existent NPM or Yarn package, npm-home will open a 404 page. When trying to open the same "package" on GitHub, npm-home throws a long error:

$ npm-home -g '~invalid~'
file:///.../npm-home/node_modules/package-json/index.js:72
			throw new PackageNotFoundError(packageName);
			      ^

PackageNotFoundError: Package `~invalid~` could not be found
    at packageJson (file:///.../npm-home/node_modules/package-json/index.js:72:10)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async openGitHub (file:///.../npm-home/cli.js:45:23)
    at async pMap.concurrency (file:///.../npm-home/cli.js:80:3)
    at async file:///.../npm-home/node_modules/p-map/index.js:139:20

This is because package-json throws:

npm-home/cli.js

Lines 45 to 46 in 7e2e51c

	const packageData = await packageJson(name, {fullMetadata: true});
	const {repository} = packageData;

This PR catches the PackageNotFoundError and logs a nicer message:

$ npm-home -g '~invalid~'
Package `~invalid~` doesn't exist!

The error message could likely be improved.