cmd/cover: Allow specifying uncovered lines

[brunokim]

In this reddit discussion, /u/jerf has a nice suggestion: label lines that should be impossible to reach to improve coverage. Something like follows:

res, err := prudentFunction("constant")
if err != nil {
  panic(fmt.Sprintf("The impossible has happened: %v", err)) // gocover: Ignore
}

The coverage tool could then instrument the line and ignore it if there are 0 passes, and issue an error if there were more than zero.

[ianlancetaylor]

CC @robpike

[robpike]

No, thanks. I dislike tools forcing people to annotate their code to silence them. That is, I dislike these annotations - they're not about the code, only about the tools that analyze the code.