fix: remove undefined variable in staticman.js

[VincentTam]

Description

The err variable in the current Staticman JS script is undefined, as my correspondence with Ricardo Fusari reveals.

Motivation and Context

The error-handling function with the error argument comes from jQuery's ajax() method. Due to @pacollins' plan to remove jQuery from the dependencies, I submitted #245 to refactor the form submission using XHR objects.

To log XHR errors, one may use myXHRobj.addEventListener('error', handleEvent) with custom JS function handleEvent(). As far as I know, few users would interest

function handleEvent {
  console.error(`${e.type}`);
  // render localized error message in HTML
}

Interested tech guys would find a way to get errors. Others might hardly need this err object

Screenshots (if appropriate):

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Checklist:

• I have updated the documentation, as applicable.
• I have updated the theme.toml, as applicable.

[VincentTam]

An screenshot illustrating the error after merging current master at d2603d7 into my demo & test site VincentTam/hugo-future-imperfect-slim@586c9c2 and switching from Utterances to Staticman.