Remove superfluous except: and change Exception returned if dnspython is not

available.

acme/acme/challenges.py

@@ -246,8 +246,8 @@ class DNS01Response(KeyAuthorizationChallengeResponse):
             txt_records = dns_resolver.txt_records_for_name(
                 validation_domain_name)
         except ImportError:  # pragma: no cover
-            raise ImportError("Local validation for 'dns-01' challenges "
-                              "requires 'dnspython'")
+            raise errors.Error("Local validation for 'dns-01' challenges "
+                               "requires 'dnspython'")
         exists = validation in txt_records
         if not exists:
             logger.debug("Key authorization from response (%r) doesn't match "

acme/acme/dns_resolver.py

@@ -19,9 +19,6 @@ def txt_records_for_name(name):
     """
     try:
         dns_response = dns.resolver.query(name, 'TXT')
-    except ImportError as error:  # pragma: no cover
-        raise ImportError("Local validation for 'dns-01' challenges requires "
-                          "'dnspython'")
     except dns.exception.DNSException as error:
         logger.error("Unable to resolve %s: %s", name, str(error))
         return []