Don't emit catching-non-exception when a tuple component has unknown bases

Close #1756

pylint/checkers/exceptions.py

@@ -305,7 +305,8 @@ class ExceptionsChecker(checkers.BaseChecker):
             if any(node is astroid.YES for node in inferred):
                 # Don't emit if we don't know every component.
                 return
-            if all(node and utils.inherit_from_std_ex(node)
+            if all(node and (utils.inherit_from_std_ex(node) or
+                             not utils.has_known_bases(node))
                    for node in inferred):
                 return

pylint/test/functional/invalid_exceptions_caught.py

@@ -121,3 +121,15 @@ try:
     raise Second
 except Second:
     pass
+
+
+class SomeBase(UnknownError):
+    pass
+
+
+EXCEPTIONS = (SomeBase, ValueError)
+
+try:
+    raise ValueError
+except EXCEPTIONS:
+    pass