It is better to have a reason string to WT_ASSERT, similar to WT_ERR_MSG to improve diagnosability.

Sometimes it is not very clear for the assertion reason.  

one example is https://github.com/mongodb/mongo/blob/e9753b56fe3f7e5c3e783f31d3ed5fe7afb8f641/src/third_party/wiredtiger/src/txn/txn.c#L1083

Instead of forcing every WT_ASSERT to accept a "reason", we can have a new construct WT_ASSERT_MSG which accepts assert condition and asserting reason.