Provide better error messages for failed remote ref resolution.

Closes #82

Author: Julian

jsonschema.py

@@ -1029,7 +1029,10 @@ def resolving(self, ref):
         elif not uri or uri == self.base_uri:
             document = self.referrer
         else:
-            document = self.resolve_remote(uri)
+            try:
+                document = self.resolve_remote(uri)
+            except Exception as exc:
+                raise RefResolutionError(exc)
 
         old_base_uri, old_referrer = self.base_uri, self.referrer
         self.base_uri, self.referrer = uri, document
@@ -1067,7 +1070,8 @@ def resolve_remote(self, uri):
         """
         Resolve a remote ``uri``.
 
-        Does not check the store first.
+        Does not check the store first, but stores the retrieved document in
+        the store if :attr:`RefResolver.cache_remote` is True.
 
         .. note::
 

tests.py

@@ -25,9 +25,10 @@
     pypy_version_info = None
 
 from jsonschema import (
-    PY3, FormatError, SchemaError, UnknownType, ValidationError, ErrorTree,
-    Draft3Validator, Draft4Validator, FormatChecker, RefResolver,
-    ValidatorMixin, draft3_format_checker, draft4_format_checker, validate,
+    PY3, FormatError, RefResolutionError, SchemaError, UnknownType,
+    ValidationError, ErrorTree, Draft3Validator, Draft4Validator,
+    FormatChecker, RefResolver, ValidatorMixin, draft3_format_checker,
+    draft4_format_checker, validate,
 )
 
 
@@ -715,6 +716,16 @@ def test_cache_remote_off(self):
             pass
         self.assertEqual(foo_handler.call_count, 2)
 
+    def test_if_you_give_it_junk_you_get_a_resolution_error(self):
+        ref = "foo://bar"
+        foo_handler = mock.Mock(side_effect=ValueError("Oh no! What's this?"))
+        resolver = RefResolver("", {}, cache_remote=True,
+                               handlers={"foo": foo_handler})
+        with self.assertRaises(RefResolutionError) as err:
+            with resolver.resolving(ref):
+                pass
+        self.assertEqual(str(err.exception), "Oh no! What's this?")
+
 
 class TestFormatChecker(unittest.TestCase):
     def setUp(self):