🚑 made exceptions nothrow-copy-constructible #531

To have nothrow-copy-constructible exceptions, we inherit from
std::runtime_error which can cope with arbitrary-length error messages.
Intermediate strings are built with static functions and then passed to
the actual constructor.

README.md

@@ -894,7 +894,7 @@ $ make json_unit -Ctest
 $ ./test/json_unit "*"
 
 ===============================================================================
-All tests passed (11202597 assertions in 47 test cases)
+All tests passed (11203022 assertions in 48 test cases)
 ```
 
 Alternatively, you can use [CMake](https://cmake.org) and run