repo_management/cli.py:
Change `db2json` and `json2db` to try/except
`errors.RepoManagementError` and `argpase.ArgumentTypeError` and exit
with a non-zero return code in that case.

tests/test_cli.py:
Change the tests for `db2json` and `json2db` to cover potentially
raising `errors.RepoManagementError` and `argpase.ArgumentTypeError` and
checking whether exiting with a non-zero return code in that case.
Add parametrized integration test `test_transform_databases()` that
transforms existing pacman .files sync databases to JSON files and back
to .files repository databases again.
Add parametrized integration
`test_transform_databases_and_use_with_pacman()` to transform existing
pacman .files sync databases to JSON files and create .files and .db
repository databases from them. Afterwards pacman uses the created
repository databases to search for packages with the "linux" keyword in
them and list the files owned by the "linux" package using the created
.files sync databases.