Descriptive error messages for julia exceptions

[jakebolewski]

Fixes segfault with PR #14.

[eslgastal]

Hi Jake,

Your changes give me the following error:

---------------------------------------------------------------------------
UnicodeDecodeError                        Traceback (most recent call last)

[...]

/usr/local/lib/python2.7/dist-packages/julia/core.pyc in _capture_showerror_for_last_julia_exception(self)
    310             sprint(showerror, ex, catch_backtrace())
    311         end""")
--> 312         return char_p(msg).value.encode("utf-8")
    313 
    314     def _typeof_julia_exception_in_transit(self):

UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)

[eslgastal]

From my understanding of julia's capi, jl_eval_string()'s return value must be unboxed to get to the actual char* returned by sprint(). This is why I originally used return self.api.jl_bytestring_ptr(msg), which doesn't segfault for me.