More informative error message when missing plot data

[fodornandor]

If output file does not have data for the selected plot a simple message is displayed e.g.:
Error: object 'pondwat' not found

Could it more informative? Like:
Error: Plot cannot be created as 'pondwat' data is missing from the output.

[fodornandor]

Using the centraldata.json, the most useful message could be sg similar (lets say harvind is missing):

Error: Plot cannot be created as '3103 Harvest Index' data is missing from the output list in the ini file.

In this way the text between the apostrophes could be copied to the ini file directly.