Added exception handling for failure to decrypt

When entering the wrong password, fernet throws an 'InvalidToken'
exception. This is now handled, but does not fully shutdown pappy.
Working on asking for password multiple times, then shutting down
completely after reasonable amount of total tries, e.g. 3.

pappyproxy/crypto.py

@@ -11,7 +11,7 @@ import twisted
 from . import compress
 from .util import PappyException
 from base64 import b64encode, b64decode
-from cryptography.fernet import Fernet
+from cryptography.fernet import Fernet, InvalidToken
 from twisted.internet import reactor, defer
 
 class Crypto(object):
@@ -78,7 +78,10 @@ class Crypto(object):
             # Decrypt the project archive
             archive_crypt = open(self.config.crypt_file, 'rb').read()
             archive_file = open(self.config.archive, 'wb')
-            archive = fern.decrypt(archive_crypt)
+            try:
+                archive = fern.decrypt(archive_crypt)
+            except InvalidToken:
+                raise PappyException("Problem decrypting the file, restart pappy to try again")
             
             archive_file.write(archive)
             archive_file.close()