fix(project): improve error message for unsupported architectures

[mr-cal]

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Fixes #4214
(CRAFT-1808)

[codecov-commenter]

Codecov Report

Merging #4392 (bd0729c) into hotfix/7.5 (061b4f0) will decrease coverage by 0.01%.
The diff coverage is 91.66%.

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@@              Coverage Diff               @@
##           hotfix/7.5    #4392      +/-   ##
==============================================
- Coverage       94.88%   94.88%   -0.01%     
==============================================
  Files             658      658              
  Lines           55138    55162      +24     
==============================================
+ Hits            52319    52341      +22     
- Misses           2819     2821       +2     

Files	Coverage Δ
snapcraft/projects.py	96.38% <100.00%> (+0.04%)	⬆️
snapcraft/utils.py	91.75% <100.00%> (+0.18%)	⬆️
tests/unit/parts/test_lifecycle.py	100.00% <ø> (ø)
tests/unit/test_projects.py	100.00% <100.00%> (ø)
tests/unit/test_utils.py	98.99% <75.00%> (-1.01%)	⬇️

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[mr-cal]

Spread test failures are unrelated

[tigarmo]

LGTM 👍