streams: Capitalize "ID" in invalid stream errors in API.

This commit changes the error message from "Invalid stream id"
to "Invalid stream ID" for cases where invalid stream IDs are
passed to API endpoints to make it consistent with other similar
error messages.

zerver/tests/test_typing.py

@@ -131,7 +131,7 @@ class TypingValidateToArgumentsTest(ZulipTestCase):
                 "topic": topic,
             },
         )
-        self.assert_json_error(result, "Invalid stream id")
+        self.assert_json_error(result, "Invalid stream ID")
 
 
 class TypingHappyPathTestPMs(ZulipTestCase):