Add logging-based error reporting

[lagru]

This refactor makes use of Python's native logging which should hopefully be a more robust foundation for future improvements such as parallelization.

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Improve output and error reporting. All output of docstub is now controllable
with the existing "--verbose" option and the new "--quiet" option. Also indicate
the (severity) level of output with a single char, e.g. "I" for information or
"W" for warnings.
{label="enhancement"}

Refactor reporting to use Python's native logging implementation.
{label="maintenance"}