feat(core): adjust coverage-report for local usage

It is now possible to run the coverage tests without moving any files
manually and the script can be called multiple times without failing.
Also improves reporting of errors.

[no changelog]

core/tools/coverage-report

@@ -1,10 +1,30 @@
 #!/bin/sh
 
-COVERAGE_THRESHOLD=${COVERAGE_THRESHOLD:-0}
+readonly COVERAGE_THRESHOLD=${COVERAGE_THRESHOLD:-0}
 
-coverage run --source=./src /dev/null 2>/dev/null
-mv .coverage .coverage.empty
-coverage combine .coverage.*
+if [ ${COVERAGE_THRESHOLD} -ne 0 ]; then
+  echo "COVERAGE_THRESHOLD set to ${COVERAGE_THRESHOLD}%"
+fi
+
+# Moving coverage files from src (tests usually save it there).
+mv -v src/.coverage* . 2>/dev/null
+
+# When there are some .coverage.* files, it is a sign
+# we need to combine them into one .coverage file.
+if ls .coverage.* 2>/dev/null; then
+  # Need to create an empty coverage file containing
+  # all existing code files, to get coverage over whole src.
+  echo "Creating .coverage.empty file"
+  coverage run --source=./src /dev/null 2>/dev/null
+  mv .coverage .coverage.empty
+  echo "Combining .coverage.* files"
+  coverage combine .coverage.*
+elif [ ! -f .coverage ]; then
+  echo "ERROR: .coverage file not found"
+  exit 1
+else
+  echo "Using already existing .coverage file"
+fi
 
 EXCLUDES="\
 src/all_modules.py,\
@@ -16,10 +36,23 @@ coverage html \
   --omit="$EXCLUDES" \
   --fail-under=${COVERAGE_THRESHOLD}
 
-if [ $? -eq 2 ]; then
-  echo "Code coverage is less than ${COVERAGE_THRESHOLD}%"
+EXIT_CODE=$?
+
+RESULT=$(grep pc_cov htmlcov/index.html | egrep -o '[0-9]{1,3}%')
+
+# Catching the case when coverage is less than threshold
+if [ ${EXIT_CODE} -eq 2 ]; then
+  echo "ERROR: Code coverage is less than ${COVERAGE_THRESHOLD}% - ${RESULT}"
+  echo "See core/htmlcov/index.html for details."
   exit 1
 fi
 
-RESULT=$(grep pc_cov htmlcov/index.html | egrep -o '[0-9]{1,3}%')
-echo "COVERAGE: ${RESULT}"
+# Catching all other errors
+if [ ${EXIT_CODE} -ne 0 ]; then
+  echo "ERROR: Coverage generation failed with exit code ${EXIT_CODE} - see output above"
+  exit ${EXIT_CODE}
+fi
+
+# Happy path
+echo "SUCCESS: COVERAGE: ${RESULT}"
+echo "See core/htmlcov/index.html for details."