Ensure bash scripts exit on fail and explain what they're doing

bin/coverage.sh

@@ -1,2 +1,5 @@
 #!/usr/bin/env bash
+set -x
+set -e
+
 pytest --cov=. --cov-report=html

bin/test.sh

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
+set -x
+set -e
 
 # Check formatting and linting
 flake8 --max-line-length 120 $(git ls-files '*.py')