Consider changing wording for traces in interactive REPL sessions

Right now, we report locations of errors in the REPL as follows:

```
In Clojure interactive session on line 1.
Call sequence:
[Clojure interactive session (repl)]
```

There was a little bit of discussion today about whether this should say `In a Clojure interactive session`, `In the Clojure interactive session`, or leave it as is (neither). We were leaning towards the second option.