Add ContainerReader utility check_container()

[pakal]

This utility loops on container data like decrypt_data() ; it checks that everything is fine in the container (proper algo codes, data structures, presence of the offladed data file near the container if needed...).

It returns a list of error messages (translatable), for all problems encountered.

[pakal]

"Create a function def check_container(container_filepath)" which does this:

• load container using load_container_from_filesystem(container_filepath, include_data_ciphertext=False)
• loop on the different parts of the file, by ASSUMING that all are of the proper type (no need for isinstance() calls)
• check that enumeration values are correct, for example that "key_encryption_algo" belongs to "ASYMMETRIC_KEY_TYPES_REGISTRY or SHARED_SECRET_MARKER", same thing for key types, signature types etc.
• when an error is encountered, add it as a properly formatted English error message to an "errors" list (e.g. something like "Invalid signature type %r encountered" % signature_type )
• return that errors list at the end

TEST this function by creating a broken encrypted container which has 1 error of each type, and checking it, to ensure that it returns a list with all proper errors.
Also test that a valid container (created with encrypt_bytestring()) returns an EMPTY list of errors.

[pakal]

This will need to use type validation tools of ticket #15

[pakal]

Security checks should be added like, number of layers in data-encryption and key-encryption

[pakal]

For now we have check_cryptainer_sanity() and check_cryptoconf_sanity(), which do not aggregate errors, but check types and enumerations properly.

What remains to be done : "Security checks should be added like, number of layers in data-encryption and key-encryption"

[pakal]

Superseded by #41