More informative error messages for `test_throws`

Before:

```
julia> using Base.Test ; @test_throws ErrorException sqrt(-1)
ERROR: test failed in expression: sqrt(-1)
 in error at error.jl:21
 in default_handler at test.jl:27
 in do_test_throws at test.jl:69
```

After:

```
julia> using Base.Test ; @test_throws ErrorException sqrt(-1)
ERROR: test failed: DomainError() was thrown instead of ErrorException
 in expression: sqrt(-1)
 in error at error.jl:21
 in default_handler at test.jl:27
 in do_test_throws at test.jl:69
```