Check if android.jar exists

I had no api 8 android.jar installed in my sdk and error message was quite confusing: 
```
Fatal Error: Unable to find package java.lang in classpath or bootclasspath
```
Now it will write:
```
A problem occurred evaluating project ':smack-android'.
> Can't find android.jar for 8 api. Please install corresponding SDK platform package
```
Please change the text at something else if you wish