Program should not crash when a json keyword is not found

[markstock]

When a json file uses old or incorrect syntax, the program crashes ungracefully. The culprit is lines in read_json functions like this:
m_thickness = j["thickness"];
One solution is to use an alternative assignment line that accepts a default value:
m_theta = j.value("rotation", 0.0);
But it would be even better to also inform the user that a property was not found and a default value was assigned.