composerr: Compose error handlers (concatenate error messages)
In labelmachine: Make Labeling of R Data Sets Easy

The functions composerr(), composerr_() and composerr_parent() modify error handlers by appending character strings to the error messages of the error handling functions:

composerr() uses non-standard evaluation.
composerr_() is the standard evaluation alternative of composerr().
composerr_parent() is a wrapper of composerr(), defining the parent environment as the lookup environment of the err_handler. This function looks up the prior error handling function in the parent environment of the current environment and allows you to store the modified error handling function under the same name as the error handling function from the parent environment without running into recursion issues. This is especially useful when doing error handling in nested environments (e.g. checking nested list objects) and you don not want to use different names for the error handling functions in the nested levels. If you don't have a nested environment situation, better use composerr() or composerr_().

composerr_(text_1 = NULL, err_prior = NULL, text_2 = NULL,
  sep_1 = ": ", sep_2 = ": ", env_prior = parent.frame())

composerr(text_1 = NULL, err_prior = NULL, text_2 = NULL,
  sep_1 = ": ", sep_2 = ": ", env_prior = parent.frame())

composerr_parent(text_1 = NULL, err_prior = NULL, text_2 = NULL,
  sep_1 = ": ", sep_2 = ": ", env_prior = parent.frame())

`text_1`	A character string, which will be appended at the beginning of the error message. The argument `sep_1` will be used as text separator.
`err_prior`	There are three valid types: `err_prior` is omitted: A new error handling message will be returned. `composerr_` is the calling function: `err_prio` must be a character string holding the name of the error handling function to which the message part should be appended. `composerr` is the calling function: `err_prio` must be the error handling function to which the message part should be appended.
`text_2`	A character string, which will be appended at the end of the error message. The argument `sep_2` will be used as text separator.
`sep_1`	A character string that is used as separator for the concatenation of `text_1` at the beginning of the error message.
`sep_2`	A character string that is used as separator for the concatenation of `text_2` at the end of the error message.
`env_prior`	An environment where the error handling function given in `err_prior` can be found. If no environment is given, then the `err_prior` will be looked up in the current environment. In the situation of nested scopes, you may change the lookup environment to the parent environment in order to be able to recursively override the name of the error handling function. In order to keep it simple, the function `composerr_parent()` can be used instead.