Fix ```writeFile``` silent fail

Writefile was missing an error callback. This was causing it to fail but give no feedback.
Merge pull request #192 from SidFerreira/patch-1

Fix writeFile silent fail
feat($cordovaBatteryStatus): add battery status support

- documentation coming next
- credits @andrewmcgivery http://goo.gl/Cby5Bv
- reference to #7
Merge pull request #194 from mrzmyr/feature-add-battery-status

feat($cordovaBatteryStatus): add battery status support