Add "could not find unit template" section to troubleshooting docs

Closes #2498, replaces #2505.
Sure, it is a bit ragged.

Since you bring it up, what *is* our standard for markup files? I tend to use 80 chars (79 max, really) for Markdown and reStructuredText since it is also intended as human-readable plan text documentation, and the lowest-common-denominator for that is the old 25x80 terminal. :older_man: But I stop short of reformatting existing documentation.