Added sticky-bit for preventing file syncs after write errors

Short story, files are no longer committed to directories during
file sync/close if the last write did not complete successfully.
This avoids a set of interesting user-experience issues related
to the end-of-life behaviour of the filesystem.

As a filesystem approaches end-of-life, the chances of running into
LFS_ERR_NOSPC grows rather quickly. Since this condition occurs after
at the end of a devices life, it's likely that operating in these
conditions hasn't been tested thoroughly.

In the specific case of file-writes, you can hit an LFS_ERR_NOSPC after
parts of the file have been written out. If the program simply continues
and closes the file, the file is written out half completed. Since
littlefs has a strong garuntee the prevents half-writes, it's unlikely
this state of the file would be expected.

To make things worse, since close is also responsible for memory
cleanup, it's actually _impossible_ to continue working as it was
without leaking memory.

By prevent the file commits, end-of-life behaviour should at least retain
a previous copy of the filesystem without any surprises.

lfs.h

@@ -75,6 +75,7 @@ enum lfs_open_flags {
     LFS_F_DIRTY   = 0x10000, // File does not match storage
     LFS_F_WRITING = 0x20000, // File has been written since last flush
     LFS_F_READING = 0x40000, // File has been read since last flush
+    LFS_F_ERRED   = 0x80000, // An error occured during write
 };
 
 // File seek flags