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, 9 insertions, 1 deletions

diff --git a/jwt/exceptions.py b/jwt/exceptions.py
index 31177a0..48a6e16 100644
--- a/jwt/exceptions.py
+++ b/jwt/exceptions.py
@@ -26,10 +26,18 @@ class ImmatureSignatureError(InvalidTokenError):
     pass
 
 
-class InvalidKeyError(Exception):
+class InvalidKeyError(ValueError):
     pass
 
 
+class InvalidAsymmetricKeyError(InvalidKeyError):
+    message = 'Invalid key: Keys must be in PEM or RFC 4253 format.'
+
+
+class InvalidJwkError(InvalidKeyError):
+    message = 'Invalid key: Keys must be in JWK format.'
+
+
 class InvalidAlgorithmError(InvalidTokenError):
     pass