Include original OAuth error in LTIExceptions

[wolever]

Currently LTIException raised in response to OAuth errors don't include the original error. This patch includes the original error and maintains the stack trace to simplify debugging.

[pdpinch]

Sorry this has been sitting unattended for so long @wolever but it's failing PEP8 tests.

/home/travis/build/mitodl/pylti/pylti/common.py:310:47: E225 missing whitespace around operator

        raise LTIException("OAuth error: %s" %(e.message, )), None, sys.exc_info()[2]

                                              ^

/home/travis/build/mitodl/pylti/pylti/common.py:310:80: E501 line too long (85 > 79 characters)

        raise LTIException("OAuth error: %s" %(e.message, )), None, sys.exc_info()[2]