fix: emit error events from <NuxtImg> placeholder

[jianxing-xu]

resolves #1639

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

I need to deal with the ERROR event when the picture is loaded, but if I use the Placehoder attribute at the same time, the ERROR event will not be thrown out

onMounted(() => {
  if (placeholder.value) {
    const img = new Image()

    if (mainSrc.value) {
      img.src = mainSrc.value
    }

    if (props.sizes) {
      img.sizes = sizes.value.sizes || ''
      img.srcset = sizes.value.srcset
    }
    // !!! Here, if the IMG loading error, there is no ERROR incident !!!
    img.onload = (event) => {
      placeholderLoaded.value = true
      emit('load', event)
    }
    // add this to fix
    img.onerror = (event) => {
      emit('error', event)
    }

    markFeatureUsage('nuxt-image')

    return
  }
  // ...other
}

[Baroshem]

Looks good to me! :)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.87%. Comparing base (25c03a4) to head (92fefef).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1640      +/-   ##
==========================================
+ Coverage   62.83%   62.87%   +0.03%     
==========================================
  Files          79       79              
  Lines        3536     3539       +3     
  Branches      412      412              
==========================================
+ Hits         2222     2225       +3     
  Misses       1285     1285              
  Partials       29       29              

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