More informative error message for era("unknown era") #28
Description
era("unknown era")
#> Error: Invalid era:
#> x `scale` must be positive
#> x `direction` must be -1 (backwards) or 1 (forwards)
#> Run `rlang::last_error()` to see where the error occurred.
Rather than trying and failing to construct an era with missing parameters, era() should recognise that the user has asked it for a predefined era that doesn't exist and give a more informative error.