exceptions: make a single-param constructor explicit

... since it's good practice.

exceptions/exceptions.hh

@@ -226,7 +226,7 @@ struct read_failure_exception : public request_failure_exception {
 };
 
 struct overloaded_exception : public cassandra_exception {
-    overloaded_exception(size_t c) noexcept :
+    explicit overloaded_exception(size_t c) noexcept :
         cassandra_exception(exception_code::OVERLOADED, prepare_message("Too many in flight hints: %lu", c)) {}
     explicit overloaded_exception(sstring msg) noexcept :
         cassandra_exception(exception_code::OVERLOADED, std::move(msg)) {}