"Data Constructor not in Scope" when defining an operator function starting with `:`

Defining a function starting with `:` is invalid according to the parsing rules:

```hs
f :< g = f . g
```

However, the error message here could see some improvement. It currently shows:

> error: Not in scope: data constructor ‘:<’

Perhaps it should instead explicitly note that only data constructors may start with colons, and function names must not.

> error: Function name may not start with a colon (':')

Or:

> error: Only data constructor names may start with a colon (':')