Inconsistent management of invalid values in BitcoinPrivateKey

[Saturnix]

BitcoinPublicKey("wrong")
result: ValueError: Public key is not in proper format

BitcoinPrivateKey("wrong")
result: AssertionError

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Explanation:
Unless there are some reasons why this has been implemented this way, I believe BitcoinPrivateKey should follow the same behavior of BitcoinPublicKey.

try/exception of invalid keys should look the same, while (as it is now) different exceptions are being presented.

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Reason:
While the file publickey.py presents a get_public_key_format function (which handles invalid formats with a ValueError), its counterpart get_privkey_format in main.py leaves this job to the function b58check_to_bin.

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Possible solution:
the call to b58check_to_bin in get_privkey_format should be wrapped in a try block, the AssertionError should be caught and a ValueError should be raised instead.

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Impact:
I have not considered the impact of such a change nor possible regressions.