Friendlier Error Messsages

## Problem

Couple reports in the discord with confusion around including a call like `App.localGetEx` return var in the seq. The error message is something like:

```
raise TealInternalError(msg) from errors[0]
pyteal.TealInternalError: Encountered 1 error when assigning slots to subroutine
```

This error message makes it difficult to figure out the actual cause

## Solution

Not sure how it could be improved

## Urgency

No urgency