HAMMER 63/Many: IO Error handling features

HAMMER 63/Many: IO Error handling features

This commit removes all the remaining Debugger() calls and KKASSERTs in the
I/O error path.  Errors are now propagated up the call tree and properly
reported.

* Report I/O errors instead of asserting.

* Read or Write errors in the flush path disable flushing and force the mount
  into read-only mode.  Modified buffers are left locked in memory until
  umount to provide a consistent snapshot of the state of the filesystem.

  You must umount and remount to recover the filesystem.  The filesystem
  will automatically rollback to the last valid flush upon remounting.

* umount and umount -f are now able to unmount a HAMMER filesystem that has
  catastrophic write errors (e.g. pulling the USB cable on an external
  drive).