Better error messages when testing arguments

[gustavdelius]

User-facing functions should not use constructs like

assert_that(
            is.data.frame(catch),
            "catch" %in% names(catch),
            "species" %in% names(catch),
            "gear" %in% names(catch),
            all(c("length", "dl") %in% names(catch)) |
                all(c("weight", "dw") %in% names(catch))
        )

because that produces cryptic error messages.