Error message content: Problem statement

Every error message should start with a general statement of the problem. It should be concise, but informative (This is hard!).

• Clear cause: use “must”
  • “n must be a numeric vector, not a character vector.”
  • “n must have length 1, not length 2.”
• Else: use “can’t”
  • “Can’t find column b in .data.”
  • “Can’t coerce .x to a vector.”
  • “Can’t find specified .depth in .x.”
• cli: Unnamed.