Added asserts on positive return values from block device functions
Added asserts on positive return values from block device functions

This has been a large source of porting errors, partially due to my
fault in not having enough porting documentation, which is also
planned.

In the short term, asserts should at least help catch these types of
errors instead of just letting the filesystem collapse after recieving
an odd error code.

geky/littlefs,geky/littlefs,littlefs-project/littlefs,geky/littlefs,littlefs-project/littlefs,geky/littlefs