Stop using GenericDialog for error messages

ImageJ has a built-in method, IJ.showMessage, for displaying messages,
including error messages, to the user.
Remove spurious GenericDialog variable

It was not used.
Add TODO reminders for duplicate code

We want to keep with the DRY principle and use method calls
instead of copy/pasted code, to avoid repeatedly fixing the same bugs.
Add underscores to GenericDialog parameters

Using GenericDialog makes macros with parameters possible. But the
parameters are derived from the label names: each "key" only spans until
the first space character (if any) of the label. So two labels that
start with the same word end up having duplicate keys; e.g.: "Export
Pixels" and  "Export Summary Histogram" and "Export Histograms" all
become the key "export" by default. To avoid this issue, replace the
spaces with underscores. ImageJ will render spaces in the label, but
then the keys will become "export_pixels" and "export_summary_histogram"
and "export_histograms" respectively.