Fix pruning IEx stacktraces too aggressively

Wrote this while trying to find a test case for https://github.com/elixir-lang/elixir/pull/2216

Before:
```
iex(1)> Code.eval_quoted(quote(do: <<((fn() -> :error end).())>>))  
** (ArgumentError) argument error
```
After:
```
iex(1)> Code.eval_quoted(quote(do: <<((fn() -> :error end).())>>))  
** (ArgumentError) argument error
    (stdlib) :erl_eval.expr/3
    (elixir) src/elixir.erl:157: :elixir.erl_eval/2
    (elixir) src/elixir.erl:150: :elixir.eval_forms/4
    (elixir) lib/code.ex:159: Code.eval_quoted/3
```