`NVM_FindRegion` should return NULL or error on failure

[k4rtik]

On a trivial program to read a string from a non-existent region, currently no error is returned:

15    uint32_t readreg = NVM_FindRegion("writer", O_RDONLY);
(gdb) 
17    assert(readreg);
(gdb) p readreg 
$1 = 100

We should perhaps return NULL and document the behavior.