Check `...` arguments in`box::file()` and `box::export()`

[Konrad Rudolph]

There’s code out in the wild that uses box::file() incorrectly by passing mustWork = FALSE to it. We should catch and report this incorrect usage by checking that the arguments have the right type and are unnamed.

• check that ... arguments are unnamed
• check that ... arguments have the right type (character for box::file(), name for box::export())