[native] Change SQL errors log level. - The SQL error itself is logged as 'info'. - The 'transaction has been rolled back' message is logged as 'debug'. These log entries should not be critical: - If an error occur at the sql level, it will be transmitted as a CW error. The exception is raised and handled in a higher level function, which will decide if it is critical or not. - Admitting that rolling-back automatically on errors is wanted in some cases, it cannot be considered 'critical'. The caller will receive the original exception anyway, it is its job to decide if it is critical or not. Closes #4801120

Added tag cubicweb-version-3.20.3, cubicweb-debian-version-3.20.3-1, cubicweb-centos-version-3.20.3-1 for changeset 7d3a583ed539

[debian] build-depend on markdown for documentation build