No validation for the charm fields when deploying

Steps to reproduce:
1. Open https://ec2-23-20-230-72.compute-1.amazonaws.com
2. On the charm browser side bar, choose a charm (e.g. mysql)
3. Click on Add
4. You are presented an option to change the service name and the number of units
5. Enter a string on the number of units field
6. Click Confirm

What happens:
You'll get an error deploying the charm

What should happen:
A nice error in the ui telling you that a string is not a valid value for the number of units.