Improve error handling osrmIsochrone(): character vector of location

[JosiahParry]

When providing a character vector of length 2 with valid coordinates the user is informed that they should provide a vector of coordinates.

informing the user that the values must be numeric would be more ergonomic / informative to the user e.g. Error: "loc" must be a numeric vector of length 2.

osrmIsochrone(c("10", "20"), breaks = seq(0, 12, 3))
#> Error: "loc" should be a vector of coordinates, i.e., c(lon, lat).

[JosiahParry]

You're quick!