The examples are not arbitrary: base64 by default ignores characters
that are not in its alphabet. When decoding, the dashes and underscores
are not b64-valid and therefore ignored, resulting in a sequence with
the wrong padding.
If the user has a wrong base64 padded secret key the googlemaps
python library is returning "TypeError: Incorrect padding" which
is very hard to understand. So now we check if the secret key is
a valid base64 string
