I did this

Added --fail-with-body to my "standard" command line curl -fsSL [...] and got curl: option --fail-with-body: is badly used here. That error message could be improved as well with the specific reason (for example by having something like a global errno variable, but containing the reason: lastArgumentParsingError = "bla bla"; return PARAM_BAD_USE;).

The man page reads This is an alternative option for -f, but does not clearly state which options are mutually exclusive.