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')`

从 internal/crypto/elliptic_curve.py 中的 _ecc_decode_compressed_point 开始，检查无效点前缀的处理方式。重现解密示例或检查显示的 traceback，然后验证未压缩点会产生带有所请求消息的 NotSupportedError，而不是 KeyError。

由索引模型根据 Issue 内容生成。