Refactor error handling in Algorithm.prepare_key() methodsfix-key-errors

Our error handling in Algorithm.prepare_key() was previously weird and
kind of inconsistent. This change makes a number of improvements:

* Refactors RSA and ECDSA prepare_key() methods to reduce nesting and
  make the code simpler to understand
* All calls to Algorithm.prepare_key() return InvalidKeyError (or a
  subclass) or a valid key instance.
* Created a new InvalidAsymmetricKeyError class that is used to provide
  a standard message when an invalid RSA or ECDSA key is used.

1 files changed, 3 insertions, 2 deletions

diff --git a/tests/contrib/test_algorithms.py b/tests/contrib/test_algorithms.py
index 6d5ca75..b774ad1 100644
--- a/tests/contrib/test_algorithms.py
+++ b/tests/contrib/test_algorithms.py
@@ -1,5 +1,6 @@
 import base64
 
+from jwt.exceptions import InvalidAsymmetricKeyError
 from jwt.utils import force_bytes, force_unicode
 
 import pytest
@@ -36,7 +37,7 @@ class TestPycryptoAlgorithms:
     def test_rsa_should_reject_non_string_key(self):
         algo = RSAAlgorithm(RSAAlgorithm.SHA256)
 
-        with pytest.raises(TypeError):
+        with pytest.raises(InvalidAsymmetricKeyError):
             algo.prepare_key(None)
 
     def test_rsa_sign_should_generate_correct_signature_value(self):
@@ -117,7 +118,7 @@ class TestEcdsaAlgorithms:
     def test_ec_should_reject_non_string_key(self):
         algo = ECAlgorithm(ECAlgorithm.SHA256)
 
-        with pytest.raises(TypeError):
+        with pytest.raises(InvalidAsymmetricKeyError):
             algo.prepare_key(None)
 
     def test_ec_should_accept_unicode_key(self):