Fix several SD card issues (#4876)

- File might not eval to false if opened with write/append and SD is gone
- Allow card to be formatted if FAT partition was not found
- Mark card as gone in certain situations
- Fix several logic errors in low level SD API

libraries/SD_MMC/src/SD_MMC.h

@@ -29,7 +29,7 @@ protected:
 
 public:
     SDMMCFS(FSImplPtr impl);
-    bool begin(const char * mountpoint="/sdcard", bool mode1bit=false);
+    bool begin(const char * mountpoint="/sdcard", bool mode1bit=false, bool format_if_mount_failed=false);
     void end();
     sdcard_type_t cardType();
     uint64_t cardSize();