Add custom class to tag loss conditions

[Bisaloo]

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• A new item has been added to NEWS.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

New feature

• What is the current behavior? (You can also link to an open issue here)

Linelist tagged variable loss condition are not classed.

• What is the new behavior (if this is a feature change)?

Customs classes (linelist_error and linelist_warning) are added to conditions raised in case of tagged variable loss.
This enables advanced condition handling, as illustrated in the following reprex:

warning_counter <- 0

withCallingHandlers({
  x <- linelist::make_linelist(cars, date_onset = "dist", age = "speed")
  x <- x[, -1]
  x <- x[, -1]
  warning("This is not a linelist warning", call. = FALSE)
}, linelist_warning = function(w) {
  warning_counter <<- warning_counter + 1
})
#> Warning: The following tags have lost their variable:
#>  age:speed
#> Warning: The following tags have lost their variable:
#>  date_onset:dist
#> Warning: This is not a linelist warning

warning("This pipeline generated ", warning_counter, " linelist warnings.")
#> Warning: This pipeline generated 2 linelist warnings.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

Fix #90