Add boundary checking

We don't want these:

Invalid number:
```
> npx rainbows -1
Here is -1 rainbow for you!
    RangeError: Invalid count value
```

Generating an incorrect number when the number specified is out of range:
```
npx rainbows 1001
Here are 1000 rainbows for you!
...
```

Add nicer error messages to these instead of crashing/outputting an incorrect number.