[Android][Extensions] Modify error message to comply with W3C spec

The spec ( http://www.w3.org/2012/sysapps/device-capabilities/ )  require an error message like:
```javascript
function(error) { console.log("error occurred: " + error.message); });
```
while the current is:
```javascript
function(error) { console.log("error occurred: " + error); });
```

The patch fix the little issue.