Unify ParamValidationError translation and document S3 error codes #1374

Part of #1342 (item 3).

Decision

1. ParamValidationError: S3Core.call raises ValueError for every operation, from the botocore error.
2. S3 error code: S3ClientError.code_of(error) is documented as the public way to get the S3 error code of a translated error. No attribute is added to the OSError.
3. exists() on 403: unchanged (True for a bucket, PermissionError for a key). It is documented, because a 403 for a key does not tell whether the key exists.
4. copy_object_annotation: closes the payload and translates the read errors.

Current state (29cc1d3, offline)

1. cat_file and pipe_file raise the raw botocore ParamValidationError; only mkdir converts it (s3.py:1387).
2. Callers use S3ClientError.code_of (s3_errors.py:139) internally; it is not documented.
3. exists(): s3.py:1108.
4. s3_core.py:1461 reads the payload without closing it, and a read failure surfaces untranslated.

Deferred: core options missing_ok for aborts and an InvalidRange mode for get_object.

Compatibility

ParamValidationError → ValueError is breaking (release note).