bma250: Improve error prints

Make sure it's possible to distinguish between the different errors
appearing due to missing or incorrectly configured sysfs nodes.

Change-Id: I7350c2243c3f1a6be7b90bda22fc933dd307fc78
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Merge pull request #27 from andersson/bma250-error-print-cleanup

Bma250 error print cleanup