ci: Correctly propagate exit status in run-style-check-diff.sh

Spotted by Daniel Stone: the addition of the echo commands in commit 65541f1a meant that the exit status from clang-format-diff.py was being lost.

The incorrect use of set +e rather than set -e meant that intermediate commands could fail without the failure being noticed.

Signed-off-by: Philip Withnall withnall@endlessm.com

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And tack on some additional commits which fix a load of shellcheck warnings.