Adding extra information about invariant messages

Author: thk123

CODING_STANDARD.md

@@ -188,6 +188,11 @@ Formatting is enforced using clang-format. For more information about this, see
 - Avoid `assert`. If the condition is an actual invariant, use INVARIANT,
   PRECONDITION, POSTCONDITION, CHECK_RETURN, UNREACHABLE or DATA_INVARIANT. If
   there are possible reasons why it might fail, throw an exception.
+    - Use "should" style statements for messages in invariants (e.g. "array
+      should have a non-zero size") to make it clear both the violation and the
+      expected behavior. (As opposed to "no zero size arrays" where it isn't
+      clear if the zero-size array is the problem, or the lack of it).
+    - The statements should start with a lower case letter.
 - All raw pointers (such as those returned by `symbol_tablet::lookup`) are
   assumed to be non-owning, and should not be `delete`d. Raw pointers that
   point to heap-allocated memory should be private data members of an object