Improve output of check_supported_type()

[krlmlr]

krlmlr@fe6bf8c#commitcomment-20176973

Perhaps include check for POSIXlt column.

@hadley: What should the error messages look like? Currently: Unsupported type CLOSXP for column "foo"

[hadley]

I'm not exactly sure but more like: Column `foo` is a function, not a vector

I'm not sure the best way of phrasing error messages:

• Column `foo` is a function, not a vector
• Column `foo` is not a vector (currently a function)
• Column `foo` must be a vector, not a function

Maybe the last one?