add explicit error if calc_addVariables() is missing some variables completely

[0UmfHxcvx5J7JoaOhFSs5mncnisTJJ6q]

Looks like

df.plot.decomb %>% 
    filter('COFFEE 1.3' != model) %>% 
    calc_addVariable(
        "`Final Energy|Industry|Iron and Steel|Residual Fossil Fuel`" =
            " `Final Energy|Industry|Iron and Steel|Solids|Coal`
            + `Final Energy|Industry|Iron and Steel|Liquids|Fossil`
            + `Final Energy|Industry|Iron and Steel|Gases|Fossil`
            + `Final Energy|Non-Energy Use|Iron and Steel|Solids|Coal`
            + `Final Energy|Non-Energy Use|Iron and Steel|Liquids|Fossil`
            + `Final Energy|Non-Energy Use|Iron and Steel|Gases|Fossil`",
        units = "EJ / yr", completeMissing = TRUE, only.new = TRUE)

Error in calc_addVariable_(data, .dots, na.rm, completeMissing, only.new, : 
3 variables are missing for the calculation:
 `Final Energy|Non-Energy Use|Iron and Steel|Solids|Coal`
 `Final Energy|Non-Energy Use|Iron and Steel|Liquids|Fossil`
 `Final Energy|Non-Energy Use|Iron and Steel|Gases|Fossil`

[nicobauer]

Thanks a lot!