Add are more meaningful message when TSV does not contain correctly named column

[matentzn]

Currently, if a dosdp cannot find a column in the tsv that it expects from the pattern definition, it says:

key not found: biological_process
Exception in thread "main" java.util.NoSuchElementException: key not found: biological_process

To be more user friendly, maybe instead just say:

TSV file does not contain the required column biological_process

[balhoff]

Columns will now be optional according to #185. I should add some warning output when the TSV is read if there are missing columns.

[matentzn]

I am slightly worried to make the columns in the vars: section of the dosdp template optional by default; Maybe I am too close to the EQ use case, but dropping the EQ because a column is misnamed that contains a filler variable seems risky. But maybe I am wrong! Would it be easy to create a switch to allow the central variables (those in the vars: section, not data_vars, or data_list_vars) to be required (or better have them required by default and only skip the requirement with that switch parameter)? Or else, make a nice and visible warning whenever a template column is not found in a TSV? What do you think @dosumis

[balhoff]

I've implemented "a nice and visible warning" in a PR. It seems like this will help avoid unexpected missing output.