fix: more edge case detection + more informative error reporting [Web mode]

[joshtrichards]

Attempt to be more informative when unexpected things happen + catch more edge cases.

Currently we return Parsing response failed when something goes wrong at any given step (since everything falls back on the JSON check). The underlying problem isn't always JSON related though. It's just our default response.

Let's be more informative and catch a few more potential edge cases while we're at it.

We should be as defensive as possible everywhere, but certainly in the Updater.

• Add handling for additional potential edge cases (i.e. loss of network connectivity or DNS)
• Add some more information to help distinguish between different error states
• Add more robust check for valid JSON
• Don't generate the summary HTML element (and useless Show detailed response button) if the details are going to be empty
• Refactor: use a common error response function (to eliminate duplicate code across the steps)
• Refactor: Misc minor (i.e. formatting)

[come-nc]

The changes should be done in index.web.php instead, and index.php generated with make index.php.