Improve error message on corrupted or tampered encrypted private key (InvalidTag)

[AlesCUMSI]

Description:
When loading a tampered or corrupted Crypt4GH private key, decryption fails with a raw cryptography.exceptions.InvalidTag exception. While this is technically correct, it’s not user-friendly and may confuse users.

Suggestion:
Catch this exception and raise a clearer message like:
"Decryption failed: The private key is corrupted or the passphrase is incorrect."

Steps to reproduce:

1. Encrypt a private key.

2. Modify two bytes in the key file (I changed values in columns 107 and 108)

3. Try to decrypt with crypt4gh decrypt.

4. The output i get:
   crypt4gh decrypt --sk ./keys-private/crypt4gh_orig_changed.key < Dockerfile_download.c4gh > Dockerfile_download_fail
   Warning: Using a passphrase in an environment variable is insecure
   Traceback (most recent call last):
   File "/home/alex/python/crypt4gh-env/bin/crypt4gh", line 8, in
   sys.exit(main())
   ^^^^^^
   File "/home/alex/python/crypt4gh-env/lib/python3.12/site-packages/crypt4gh/main.py", line 21, in main
   cmd(args)
   File "/home/alex/python/crypt4gh-env/lib/python3.12/site-packages/crypt4gh/cli.py", line 173, in decrypt
   seckey = retrieve_private_key(args)
   ^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/alex/python/crypt4gh-env/lib/python3.12/site-packages/crypt4gh/cli.py", line 125, in retrieve_private_key
   return get_private_key(seckeypath, cb)
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/alex/python/crypt4gh-env/lib/python3.12/site-packages/crypt4gh/keys/init.py", line 122, in get_private_key
   return c4gh.parse_private_key(stream, callback)
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/home/alex/python/crypt4gh-env/lib/python3.12/site-packages/crypt4gh/exceptions.py", line 39, in wrapper
   return func(*args, **kwargs)
   ^^^^^^^^^^^^^^^^^^^^^
   File "/home/alex/python/crypt4gh-env/lib/python3.12/site-packages/crypt4gh/keys/c4gh.py", line 132, in parse_private_key
   return ChaCha20Poly1305(shared_key).decrypt(nonce, encrypted_data, None) # No add
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   cryptography.exceptions.InvalidTag

Thanks for the great work!

[silverdaz]

Sure!
Maybe a nice friendly message, and when in debug/verbose mode, we output this longer message.

Do you want to give it a try and make a PR ?