Downgraded express-hbs errors to 400

[ErisDS]

refs: https://github.com/TryGhost/Team/issues/2289 refs: TryGhost/express-hbs#161

• Themes that resuse layouts as templates trigger horrible errors, which are thrown as 500s
• But there's nothing the server is doing wrong, it's a theme user, so we downgrade these to 400s
• There is more to do here to improve the errors shown, but this is just a first step to ensure that theme issues don't look like server failures

[codecov]

Codecov Report

Base: 67.76% // Head: 67.76% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (6271750) compared to base (5457402).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15864      +/-   ##
==========================================
- Coverage   67.76%   67.76%   -0.01%     
==========================================
  Files        1620     1620              
  Lines      105114   105130      +16     
  Branches    14958    14959       +1     
==========================================
+ Hits        71231    71240       +9     
- Misses      32602    32610       +8     
+ Partials     1281     1280       -1     

Flag	Coverage Δ
e2e-tests	76.59% <ø> (-0.01%)	⬇️
unit-tests	53.72% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
ghost/mw-error-handler/lib/mw-error-handler.js	100.00% <100.00%> (ø)
...ata-generator/lib/tables/members-created-events.js	85.71% <0.00%> (-8.58%)	⬇️
...host/data-generator/lib/tables/members-products.js	97.14% <0.00%> (-2.86%)	⬇️
ghost/admin/app/components/gh-image-uploader.js	84.35% <0.00%> (-1.37%)	⬇️
ghost/core/core/server/services/mega/mega.js	60.49% <0.00%> (-0.54%)	⬇️
ghost/admin/app/controllers/offer.js	42.63% <0.00%> (-0.53%)	⬇️
ghost/core/core/server/models/user.js	87.56% <0.00%> (ø)
ghost/core/core/frontend/helpers/tiers.js	93.18% <0.00%> (ø)
ghost/core/core/server/models/settings.js	90.57% <0.00%> (ø)
ghost/core/core/frontend/helpers/post_class.js	89.74% <0.00%> (ø)
... and 12 more

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