Handle missing dnspython by displaying a warning message

acme/acme/challenges.py

@@ -234,8 +234,8 @@ class DNS01Response(KeyAuthorizationChallengeResponse):
         try:
             from acme import dns_resolver
         except ImportError:  # pragma: no cover
-            raise errors.Error("Local validation for 'dns-01' challenges "
-                               "requires 'dnspython'")
+            raise errors.DependencyError("Local validation for 'dns-01' "
+                                         "challenges requires 'dnspython'")
         txt_records = dns_resolver.txt_records_for_name(validation_domain_name)
         exists = validation in txt_records
         if not exists:

acme/acme/errors.py

@@ -6,6 +6,10 @@ class Error(Exception):
     """Generic ACME error."""
 
 
+class DependencyError(Error):
+    """Dependency error"""
+
+
 class SchemaValidationError(jose_errors.DeserializationError):
     """JSON schema ACME object validation error."""