expose ff.c constants, raise them as arguments to FatFSError
introduce NotMounted and NoFilesystem as subclasses of FatFSError with
the appropriate error code set
It is possible to call `ensure_sdcard` in a way that requires only SD
card be inserted, but not necessarily formatted.
This is useful for SD-protect and possibly other use-cases where the SD
card is read-only, and "not formatted" is identical to "not containing
the right files".
