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.

[codecov-commenter]

Codecov Report

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

Project coverage is 82.54%. Comparing base (45038ea) to head (8470730). Report is 1 commits behind head on 1.2-maint.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 1.2-maint #8211 +/- ## ============================================= - Coverage 82.55% 82.54% -0.01% ============================================= Files 38 38 Lines 10838 10838 Branches 2079 2079 ============================================= - Hits 8947 8946 -1 - Misses 1340 1341 +1 Partials 551 551 ```

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