export user-visible exceptions in primary module

since MalformedPointError and UnexpectedDER are user-visible (thrown
by methods that users are expected to use), export them in the root
of the module hierarchy

Author: tomato42

README.md

@@ -123,7 +123,8 @@ is to call `s=sk.to_string()`, and then re-create it with
 `SigningKey.from_string(s, curve)` . This short form does not record the
 curve, so you must be sure to tell from_string() the same curve you used for
 the original key. The short form of a NIST192p-based signing key is just 24
-bytes long.
+bytes long. If a point encoding is invalid or it does not lie on the specified
+curve, `from_string()` will raise MalformedPointError.
 
 ```python
 from ecdsa import SigningKey, NIST384p
@@ -139,7 +140,8 @@ formats that OpenSSL uses. The PEM file looks like the familiar ASCII-armored
 is a shorter binary form of the same data.
 `SigningKey.from_pem()/.from_der()` will undo this serialization. These
 formats include the curve name, so you do not need to pass in a curve
-identifier to the deserializer.
+identifier to the deserializer. In case the file is malformed `from_der()`
+and `from_pem()` will raise UnexpectedDER or MalformedPointError.
 
 ```python
 from ecdsa import SigningKey, NIST384p

src/ecdsa/__init__.py

@@ -1,5 +1,7 @@
-from .keys import SigningKey, VerifyingKey, BadSignatureError, BadDigestError
+from .keys import SigningKey, VerifyingKey, BadSignatureError, BadDigestError,\
+        MalformedPointError
 from .curves import NIST192p, NIST224p, NIST256p, NIST384p, NIST521p, SECP256k1
+from .der import UnexpectedDER
 
 # This code comes from https://github.com/warner/python-ecdsa
 from ._version import get_versions
@@ -10,5 +12,6 @@
            "test_pyecdsa", "util", "six"]
 
 _hush_pyflakes = [SigningKey, VerifyingKey, BadSignatureError, BadDigestError,
+                  MalformedPointError, UnexpectedDER,
                   NIST192p, NIST224p, NIST256p, NIST384p, NIST521p, SECP256k1]
 del _hush_pyflakes