Descriptive error messages for julia exceptions

[eslgastal]

This patch brings more descriptive error messages when julia code throws an exception. Example output in IPython notebook:

[stevengj]

Would be simpler to just do sprint(showerror, e, catch_backtrace())

[jakebolewski]

Hmm this segfaults for me (a is undefined). The old version worked in this example.

In [5]: jenv.eval("1 / a")

signal (11): Segmentation fault: 11
jl_bytestring_ptr at /private/tmp/julia-krtFXw/src/jlapi.c:147
ffi_call_unix64 at /usr/local/Cellar/python/2.7.8_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload/_ctypes.so (unknown line)
ffi_call at /usr/local/Cellar/python/2.7.8_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload/_ctypes.so (unknown line)
_ctypes_callproc at /usr/local/Cellar/python/2.7.8_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload/_ctypes.so (unknown line)
PyCFuncPtr_call at /usr/local/Cellar/python/2.7.8_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload/_ctypes.so (unknown line)
PyObject_Call at /usr/local/Cellar/python/2.7.8_1/Frameworks/Python.framework/Versions/2.7/Python (unknown line)
...

[jakebolewski]

@egastal I fixed the problem I was having. These error messages are much better, thanks!