Some fixes for `-Wall`-readiness:
* Removed `using namespace std;` from the header and
added `std::` prefixes where needed (`std::string`)
* Replaced `throw ()` in declarations with `noexcept`,
as `throw ()` is [deprecated in c++11][1]
* Several exception classes had a `file_path` member
variable, despite never using it. Removed the
unused class member.
[1]:<https://en.cppreference.com/w/cpp/language/noexcept_spec>
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