fix(integ-runner): fail with correct error when integ manifest exists but cannot be loaded

[mrgrain]

Previously if an integ manifest exists, but could not be loaded for some reason we would be falling back to the deprecated legacy style tests. This code path will than always fail with a misleading error: "integ-runner" can only operate on apps with a single stack. The error is because the modern integ tests always have include stacks, but legacy tests only handle a single stack or a pragma list.

This change improves the error handling to distinguish between "no integ manifest" and "failed to load manifest". For the former, we still go down the legacy test route. For the latter we can now fail the test with an accurate error message.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.41%. Comparing base (19f3b5e) to head (09f8115).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #692   +/-   ##
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  Coverage   79.41%   79.41%           
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  Files          49       49           
  Lines        7358     7358           
  Branches      818      818           
=======================================
  Hits         5843     5843           
  Misses       1496     1496           
  Partials       19       19           

Flag	Coverage Δ
suite.unit	79.41% <ø> (ø)

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