setup.py: fix import error reporting for cythonize import, see #8208

[ThomasWaldmann]

Looks like borg's setup.py has hidden the real cause of a cythonize ImportError.

There are basically 2 cases:

• either there is no Cython installed, then the import fails because the module can not be found, or
• there is some issue within Cython and the import fails due to that.

It's important not to hide the real cause, especially if we run into case 2.

case 1 is kind of expected and frequent, case 2 is rare.

[ThomasWaldmann]

The macOS failure is due to:

  ld: library 'crypto' not found

Maybe some OpenSSL change in the action runner?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.47%. Comparing base (b961792) to head (8dcd8da). Report is 3 commits behind head on 1.4-maint.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 1.4-maint #8210 +/- ## ========================================== Coverage 82.47% 82.47% ========================================== Files 38 38 Lines 11027 11027 Branches 2116 2116 ========================================== Hits 9094 9094 Misses 1365 1365 Partials 568 568 ```

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