Non-existent firmware will be upload, need to check it it exists before trying to flash

[mineshaftgap]

In manifest.json if there is a typo, self-hosted ESP Web Tools will try and upload a non-existent firmware resulting in a boot loop of:

invalid header: 0x434c4557

More details can be found in a previously related ticket. I suggest that checking for the existence of the file before trying to upload or in that is not possible to do a file size check.

[balloob]

We check if the status code is correct:

esp-web-tools/src/flash.ts

Lines 119 to 124 in ec35b6a

	const resp = await fetch(url);
	if (!resp.ok) {
	throw new Error(
	`Downlading firmware ${part.path} failed: ${resp.status}`,
	);
	}

Web servers should return a 404 status code if the file is not found. If that is not the case, your web server is misconfigured.