Add optional success message to rules

[k-dauda]

Currently only error messages are displayed, however, it may be sometimes be desirable to give positive feedback to users some examples include:

• Password (indicate how good password is)
• Username field (indicate username is available)

[jbandura]

That's a good idea 👍 If you would like to work on this, I'd be happy to provide any assistance needed :) Otherwise I'll try to get to it soonish