Impossible to define/raise user-defined exceptions

This works:
```
data Error = Fail

main = Raise Subscript
```

While the following is rejected by the compiler:
```
data Error = Fail

main = Raise Fail
```

Start by compiling the two examples in the issue and compare the accepted Raise Subscript case with the rejected Raise Fail case. Trace how user-defined exception constructors are handled, then confirm that Raise Fail is accepted while the existing behavior remains intact.

Written by the indexing model from the issue text.