Better schema validation

[hkage]

If a schema has an invalid structure, the resulting errors are a bit confusing. E.g. if the "tables" definition is missing, the resulting error looks like this:

Traceback (most recent call last):
  File "/home/henning/.local/share/virtualenvs/postgresql-anonymizer-AUSqld0C/bin/pganonymize", line 11, in <module>
    load_entry_point('pganonymize', 'console_scripts', 'pganonymize')()
  File "/home/henning/Projekte/postgresql-anonymizer/pganonymizer/cli.py", line 32, in main
    truncate_tables(connection, schema.get('truncate', []))
AttributeError: 'list' object has no attribute 'get'

It would ne nice to have a validation method that checks for the basic structure of the YAML file:

• The tables keyword
• fields at the table level
• A provider at the field level, with at least a name
• ...

Instead of writing own methods for the validation it could be possible to use general Python based YAML validation libraries, e.g.:

• https://github.com/pyeve/cerberus/
• https://github.com/23andMe/Yamale (only Python 3.6+)
• https://github.com/keleshev/schema