fix: a pretty error when browser executable is not found

[aslushnikov]

Make sure executable exists before launching it. If it doesn't and
we were launched without custom executable path, print a helpful
instruction to run npm i playwright and get browsers downloaded.

Note: there's already a test that makes sure bad executable paths
are treated fairly:

playwright/test/launcher.jest.js

Lines 54 to 59 in 9132d23

	it('should reject if executable path is invalid', async({browserType, defaultBrowserOptions}) => {
	let waitError = null;
	const options = Object.assign({}, defaultBrowserOptions, {executablePath: 'random-invalid-path'});
	await browserType.launch(options).catch(e => waitError = e);
	expect(waitError.message).toContain('Failed to launch');
	});

This doesn't test missing default browser installation which I think is
fine.

Fixes #3161