Shiny Updates: Enhance error reporting when response is invalid

[ocean90]

wp.updates.isValidResponse() checks the Ajax response and tries to provide a meaningful error message in case of an error. This doesn't work always:

• Invalid nonce: "Update Failed: -1"
• Connection lost: "Update Failed: error" (see #37550 for a screenshot)
• 404 error: "Update Failed: $html" (​https://cloudup.com/c-2MST8o_RE, unescaped in the admin notice)

[ocean90]

37583.patch​:

• Escape messages for admin notices
• Display "Update Failed: An error has occurred. Please reload the page and try again." for a nonce failure.
• Display "Update Failed: Connection lost or the server is busy. Please try again later." for connection lost.
• Both strings are already used by the editor/media components.

[ocean90]

A 404 seems to be unlikely, it would mean that admin-ajax.php is gone. So the raw output should be fine here.

[azaozz]

37583.patch looks good but we may need to strip HTML from the passed error messages. Got this by adding sleep(31) to wp-config.php

Update Failed: <br /> <b>Fatal error</b>: Maximum execution time of 30 seconds exceeded in <b>\test\wp-config.php</b> on line <b>45</b><br />

[ocean90]

In 38205:

Update/Install: Replace "error" and "-1" failure messages with a more meaningful one.

• "-1" is an invalid nonce error, show 'An error has occurred. Please reload the page and try again.'.
• "error" means that the connection to the server was lost, show 'Connection lost or the server is busy. Please try again later.'.
• Escape the message in wp-updates-admin-notice because the response may include HTML.
• Remove HTML tags in wp.updates.isValidResponse() to make PHP's error messages more readable.

Props azaozz for review.
Fixes #37583.

[ocean90]

In 38206:

Update/Install: Replace "error" and "-1" failure messages with a more meaningful one.

• "-1" is an invalid nonce error, show 'An error has occurred. Please reload the page and try again.'.
• "error" means that the connection to the server was lost, show 'Connection lost or the server is busy. Please try again later.'.
• Escape the message in wp-updates-admin-notice because the response may include HTML.
• Remove HTML tags in wp.updates.isValidResponse() to make PHP's error messages more readable.

Merge of [38205] to the 4.6 branch.

Props azaozz for review.
See #37583.