Add some noexcepts

This is to assert that callback functions should never throw (since
the context in which they're called may not be able to handle the
exception).

src/libstore/http-binary-cache-store.cc

@@ -131,7 +131,7 @@ protected:
     }
 
     void getFile(const std::string & path,
-        Callback<std::shared_ptr<std::string>> callback) override
+        Callback<std::shared_ptr<std::string>> callback) noexcept override
     {
         checkEnabled();