drivers/nble: Fix handling read errors

If on_nble_gattc_read_rsp returns an error, this error shall be
passed to the application to let know that Read Request/Read Blob Request
was aborted due to ATT error received.

Change-Id: I0efe26ec90fe27a1ab3c791e555ebe682927ab57
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>