Improve lua error reporting when by accident variables were used

[Broes De Cat]

Currently, errors only state what type is expected. Often it is also useful to know what type was supplied. For example, when a string was expected, but a user forgot the quotes, resulting in a non-existent variable, the error is error ... string expected

while it would be useful to know "nil" was supplied.

Also a problem which happens often in stdoptions: stdoptions.language=ecnf should be stoptions.language="ecnf" but very difficult to find the cause