User assist modals - provide specialized help for problematic cases

[grumpyinca]

Different error conditions can present different levels of difficulty for users to cope with. The
"Search/Seek cannot continue because there are no free independent variables. Help button provides more information." message seems to be particularly problematic for some users.

Various messages (displayMessage, ResultTable termination_condition, etc.) should be examined to determine which should get a dedicated modal that supplies specialized help for that situation.

For example, in the case of no free independent variables, the special modal could provide words like "You must free (uncheck) at least one independent variable to continue with Search / Seek / Trade" and then present a list of all independent variables with checkboxes.