Print original stacktrace on debug errors

[jamesls]

Before:

{
    "errorMessage": "Raising an exception!",
    "errorType": "RuntimeError",
    "stackTrace": [
        [
            "/var/task/chalice/__init__.py",
            198,
            "__call__",
            "raise e"
        ]
    ]
}

After:

{
    "errorMessage": "Raising an exception!",
    "errorType": "RuntimeError",
    "stackTrace": [
        [
            "/var/task/chalice/__init__.py",
            192,
            "__call__",
            "response = view_function(*function_args)"
        ],
        [
            "/var/task/app.py",
            9,
            "index",
            "return a()"
        ],
        [
            "/var/task/app.py",
            12,
            "a",
            "return b()"
        ],
        [
            "/var/task/app.py",
            15,
            "b",
            "raise RuntimeError(\"Raising an exception!\")"
        ]
    ]
}

This also shows up in chalice logs:

2016-08-01 16:14:06.897000 dcd6c3 Raising an exception!: RuntimeError
Traceback (most recent call last):
  File "/var/task/chalice/__init__.py", line 192, in __call__
    response = view_function(*function_args)
  File "/var/task/app.py", line 9, in index
    return a()
  File "/var/task/app.py", line 12, in a
    return b()
  File "/var/task/app.py", line 15, in b
    raise RuntimeError("Raising an exception!")
RuntimeError: Raising an exception!

Fixes #50.

[woodlandh]

Thanks for implementing this @jamesls !