BpfMap.h - simplify error returns

Returning the number of the file descriptor isn't actually useful since
it doesn't really tell you anything once the program has terminated.
(most logs we look at are long after the fact)

However, it does involve a fair bit of string processing to generate
the errors.  This is particularly an issue for things we constantly
call and sometimes expect to fail.  For example it is normal for
getNextKey() to fail with ENOENT at the end of iteration...

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ib94037440b5fcaa34cc8aa01b165431efabeb377