Fix coverage report merging

For some time now (I'd guess almost a year 🤷), the coverage
merging on Windows has been failing, because the reports have been
generated in a different folder than expected. Our merge script did
not report failure because it was not checking the returned error
code from OpenCppCoverage, and for some reason, the `codecov` tool
happily returned 0 even though it did not find the file it was
supposed to upload...

The former is also fixed by this commit.

misc/appveyorMergeCoverageScript.py

@@ -4,6 +4,6 @@ import glob
 import subprocess
 
 if __name__ == '__main__':
-    cov_files = list(glob.glob('cov-report*.bin'))
+    cov_files = list(glob.glob('projects/cov-report*.bin'))
     base_cmd = ['OpenCppCoverage', '--quiet', '--export_type=cobertura:cobertura.xml'] + ['--input_coverage={}'.format(f) for f in cov_files]
-    subprocess.call(base_cmd)
+    subprocess.check_call(base_cmd)