Enhance tracebacks.

[JulienPalard]

Seen a traceback like this:

$ python3 -c 'import jwt; jwt.decode("foo")'
/home/mdk/clones/pyjwt/jwt/api_jwt.py:88: DeprecationWarning: It is strongly recommended that you pass in a value for the "algorithms" argument when calling decode(). This argument will be mandatory in a future version.
  warnings.warn(
Traceback (most recent call last):
  File "/home/mdk/clones/pyjwt/jwt/api_jws.py", line 189, in _load
    signing_input, crypto_segment = jwt.rsplit(b".", 1)
ValueError: not enough values to unpack (expected 2, got 1)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/mdk/clones/pyjwt/jwt/api_jwt.py", line 95, in decode
    payload, _, _, _ = self._load(jwt)
  File "/home/mdk/clones/pyjwt/jwt/api_jws.py", line 192, in _load
    raise DecodeError("Not enough segments")
jwt.exceptions.DecodeError: Not enough segments

so I just added some raise ... from ... to enhance the message to:

/home/mdk/clones/pyjwt/jwt/api_jwt.py:88: DeprecationWarning: It is strongly recommended that you pass in a value for the "algorithms" argument when calling decode(). This argument will be mandatory in a future version.
  warnings.warn(
Traceback (most recent call last):
  File "/home/mdk/clones/pyjwt/jwt/api_jws.py", line 189, in _load
    signing_input, crypto_segment = jwt.rsplit(b".", 1)
ValueError: not enough values to unpack (expected 2, got 1)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/mdk/clones/pyjwt/jwt/api_jwt.py", line 95, in decode
    payload, _, _, _ = self._load(jwt)
  File "/home/mdk/clones/pyjwt/jwt/api_jws.py", line 192, in _load
    raise DecodeError("Not enough segments") from err
jwt.exceptions.DecodeError: Not enough segments

[coveralls]

Pull Request Test Coverage Report for Build 203

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 132:	0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[auvipy]

Thanks, Jullen!