raise a better error when decryption of a message with an uncompressed EC point is attempted

## Problem

In testing to verify what happens if a caller attempts to decrypt a file created by [mrcrypt](https://github.com/aol/mrcrypt), I realized that if you attempt to decrypt a message that was written with an uncompressed EC point (note: this is not supported under the AWS Encryption SDK spec), you get a very unhelpful error.

```
$ mrcrypt encrypt alias/exampleKey test_plaintext -r us-west-2
$ python
>>> import aws_encryption_sdk
>>> mkp = aws_encryption_sdk.KMSMasterKeyProvider()
>>> with open('test_plaintext.encrypted', 'rb') as f:
... ct = f.read()
...
>>> aws_encryption_sdk.decrypt(source=ct, key_provider=mkp)
No handlers could be found for logger "aws_encryption_sdk.streaming_client"
Traceback (most recent call last):
File "", line 1, in
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/__init__.py", line 121, in decrypt
plaintext = decryptor.read()
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/streaming_client.py", line 201, in read
self._prep_message()
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/streaming_client.py", line 687, in _prep_message
self._header, self.header_auth = self._read_header()
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/streaming_client.py", line 719, in _read_header
decryption_materials = self.config.materials_manager.decrypt_materials(request=decrypt_materials_request)
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/materials_managers/default.py", line 149, in decrypt_materials
encryption_context=request.encryption_context
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/materials_managers/default.py", line 129, in _load_verification_key_from_encryption_context
encoded_point=encoded_verification_key
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/internal/crypto/authentication.py", line 159, in from_encoded_point
compressed_point=base64.b64decode(encoded_point)
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/internal/crypto/elliptic_curve.py", line 182, in _ecc_public_numbers_from_compressed_point
x, y = _ecc_decode_compressed_point(curve, compressed_point)
File "/Users/bullocm/tmp/mrt/lib/python2.7/site-packages/aws_encryption_sdk/internal/crypto/elliptic_curve.py", line 140, in _ecc_decode_compressed_point
y_order = y_order_map[raw_y]
KeyError: '\x04'
```

## Solution
Catch appropriate errors in `internal.crypto.elliptic_curve._ecc_decode_compressed_point` and raise `NotSupportedError('Uncompressed points are not supported')`

Beginne in internal/crypto/elliptic_curve.py bei _ecc_decode_compressed_point und überprüfe, wie ungültige Punktpräfixe behandelt werden. Reproduziere das Entschlüsselungsbeispiel oder untersuche den gezeigten Traceback und verifiziere anschließend, dass ein unkomprimierter Punkt NotSupportedError mit der angeforderten Meldung statt KeyError erzeugt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.