Print global name in error messages

[balazsdukai]

If a global has an empty default value set in the flowchart (input_footprints=""), then running the flowchart brings an error message with an empty string. This makes it quite difficult to find where there error comes from.

$ geof single/reconstruct.json

ERROR: Error: Open failed on 

The error message should also print the global name, not only the value. Sth like

$ geof single/reconstruct.json

ERROR: Open failed on input_footprints

or name + value

$ geof single/reconstruct.json

ERROR: Open failed on input_footprints=