calc_limits with more informative error messaging

[sebdalgarno]

The function does not give any indication to user why limits were not calculated...simply tells them that values were dropped.

Error messaging should be as user-friendly as possible, because they will be captured by the shiny app and displayed verbatim to the user.

[sebdalgarno]

for extra context, the shiny app currently displays a single message if the wqbc::calc_limits returns an error or a data.frame with 0 rows.
"There is insufficient data to calculate the water quality guideline. You could try to get a modelled estimate, change the term, or set the guideline manually."

The team want to know more about what's happening behind the scenes...is there insufficient data for that term argument? is there no limit for that Variable?

[sebdalgarno]

related to bcgov/shinyrems#74