JsonableError: Rename status_code and rely more on its default.

With #5598 there will soon be an application-level error code
optionally associated with a `JsonableError`, so rename this
field to make clear that it specifically refers to an
HTTP status code.

Also take this opportunity to eliminate most of the places
that refer to it, which only do so to repeat the default value.

zerver/views/streams.py

@@ -35,10 +35,10 @@ from six.moves import urllib
 import six
 
 class PrincipalError(JsonableError):
-    def __init__(self, principal, status_code=403):
-        # type: (Text, int) -> None
+    def __init__(self, principal):
+        # type: (Text) -> None
         self.principal = principal  # type: Text
-        self.status_code = status_code  # type: int
+        self.http_status_code = 403  # type: int
 
     def to_json_error_msg(self):
         # type: () -> Text