WIP Refactoring of exceptions:

1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.

src/slic3r/Utils/Http.cpp

@@ -156,7 +156,7 @@ Http::priv::priv(const std::string &url)
     Http::tls_global_init();
     
 	if (curl == nullptr) {
-		throw std::runtime_error(std::string("Could not construct Curl object"));
+		throw Slic3r::RuntimeError(std::string("Could not construct Curl object"));
 	}
 
 	set_timeout_connect(DEFAULT_TIMEOUT_CONNECT);