matchers/throw_exception: Declare type of expected_exception
matchers/throw_exception: Declare type of expected_exception

This is to improve readability and to understand better
a weird error of Visual Studio 14 2015.
Most interestingly, this fixes the error.

joakimkarlsson/bandit,ogdf/bandit,joakimkarlsson/bandit,ogdf/bandit,joakimkarlsson/bandit