[Shipping Labels Revamp] Add validations and error handling to Customs form

[ThomazFB]

Closes: #

Description

Steps to reproduce

Testing information

The tests that have been performed

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• I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

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• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on big (tablet) and small (phone) in case of UI changes, and no regressions are added.

[dangermattic]

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce-Wear Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	415034e
	Direct Download	woocommerce-wear-prototype-build-pr13465-415034e.apk

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	415034e
	Direct Download	woocommerce-prototype-build-pr13465-415034e.apk

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.47059% with 8 lines in your changes missing coverage. Please review.

Project coverage is 37.89%. Comparing base (3f3a643) to head (415034e).
Report is 19 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...glabels/customs/WooShippingCustomsFormViewModel.kt	76.47%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk   #13465      +/-   ##
============================================
+ Coverage     37.88%   37.89%   +0.01%     
- Complexity     8956     8961       +5     
============================================
  Files          2050     2050              
  Lines        112105   112132      +27     
  Branches      14173    14180       +7     
============================================
+ Hits          42474    42496      +22     
- Misses        65751    65754       +3     
- Partials       3880     3882       +2     

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[wpmobilebot]

Version 21.7 has now entered code-freeze, so the milestone of this PR has been updated to 21.8.