warn user if they hadn't changed API key

[nshadov]

Display more understandable/trackable error message about missing API key:

...
File "./vtlite.py", line 15, in __init__
    assert "KEY-GOES-HERE" not in your_public_api_key
AssertionError

[justNik101]

Just include it in try except block (enhance it) like -
try:
your_private_api_key = '<--------------PRIVATE-API-KEY-GOES-HERE----->'
assert "KEY-GOES-HERE" not in your_private_api_key
except AssertionError:
print("Oops! it looks like you forgot to add your API key!")

[justNik101]

Great, it can be more better, add one more condition to check if Api variable has same length as Api keys and it is in hexadecimal format