Propagate body decode errors instead of swallowing them (#24)

[kurok]

Summary

Fixes issue #24 (high-priority bug): body decode errors were silently swallowed.

The MIME parts loop in src/mail_parser.rs decoded each part body with get_body_raw().unwrap_or_default() and get_body().unwrap_or_default(). When mailparse failed to decode a body (broken base64/quoted-printable or an undecodable charset), the error was discarded and an empty value stored — hiding corruption from the caller.

Change

• Propagate the MailParseError from get_body_raw/get_body up through Mail::new (which already returns Result<_, MailParseError>). The PyO3 layer already maps MailParseError to the Python ParseError, so callers now see decode failures rather than silently-empty bodies.
• No behavior change for valid emails (including valid base64): the full existing suite (87 tests) still passes unchanged.

Tests

Added tests/test_decode_errors.py:

• broken-base64 text body raises ParseError (the get_body() path)
• broken-base64 named attachment raises ParseError (the get_body_raw() path)
• valid base64 body still parses
• plain unencoded message still parses

Full suite: 91 passed (87 existing + 4 new), pytest -q --ignore tests/benchmark tests.

Note: quoted-printable decoding in mailparse 0.15 is tolerant of invalid sequences (it passes them through rather than erroring), so the deterministic broken-encoding tests use base64.

Closes #24