Set found to false if any paths are not found

Many unix utilities exit with a code of 1 if any parts of the command fail, in particular `rpm`.  I believe its desirable for gem to behave similar to `rpm -q` for this use case.

```
# rpm -q man
man-1.6f-32.el6.x86_64
# echo $?
0
```

```
# rpm -q man cheese
man-1.6f-32.el6.x86_64
package cheese is not installed
# echo $?
1
```