When there is no wheel in %{_pyproject_wheeldir}, %pyproject_install fails (expected), but the failure is quite cryptic:

...
+ cd Pello-1.0.4
++ ls '/builddir/build/BUILD/Pello-1.0.4/pyproject-wheeldir/*.whl'
++ xargs basename --multiple
++ sed -E 's/([^-]+)-([^-]+)-.+\.whl/\1==\2/'
ls: cannot access '/builddir/build/BUILD/Pello-1.0.4/pyproject-wheeldir/*.whl': No such file or directory
basename: missing operand
Try 'basename --help' for more information.
+ specifier=
+ TMPDIR=/builddir/build/BUILD/Pello-1.0.4/.pyproject-builddir
+ /usr/bin/python3 -m pip install --root /builddir/build/BUILDROOT/python-pello-1.0.4-1.fc40.x86_64 --prefix /usr --no-deps --disable-pip-version-check --progress-bar off --verbose --ignore-installed --no-warn-script-location --no-index --no-cache-dir --find-links /builddir/build/BUILD/Pello-1.0.4/pyproject-wheeldir
Using pip 23.2.1 from /usr/lib/python3.12/site-packages/pip (python 3.12)
ERROR: You must give at least one requirement to install (maybe you meant "pip install /builddir/build/BUILD/Pello-1.0.4/pyproject-wheeldir"?)
error: Bad exit status from /var/tmp/rpm-tmp.1kfx77 (%install)


As you can see, the last relevant error message is:

  ERROR: You must give at least one requirement to install (maybe you meant "pip install /builddir/build/BUILD/Pello-1.0.4/pyproject-wheeldir"?)

This is of course not up to the user of our macros, they don't invoke pip directly. The actual problem is hidden a couple of lines earlier:

  ls: cannot access '/builddir/build/BUILD/Pello-1.0.4/pyproject-wheeldir/*.whl': No such file or directory
  basename: missing operand

This causes our shell code in the macro to pass no positional argument to pip install.

We should explicitly fail when this happens with an actionable error message.

Reproducible: Always

Steps to Reproduce:
1. Take the example specfile from Python packaging guidelines: https://docs.fedoraproject.org/en-US/packaging-guidelines/Python/#_example_spec_file
2. Remove the %pyproject_wheel line
3. Build the package
Actual Results:  
Build fails, the error message is cryptic and not actionable, actual error buried in the log.

Expected Results:  
Build fails, the error message is understandable and actionable, the error message is as late in the log as possible.