Improvements/fixes on the workflow to create organizations

[pcrespov]

• Following up from ♻️ Enhanced groups/organizations web-api specs and validation 🚨 #6640, these are small issues that we can improve in the workflow to create organizations

Here some issues found in the workflow:

1. when clicking "New Organization" the window opens and I would like to have the cursor in Title right away
   
2. Improve error handling.
   • in-place validation of url or use error from backend
     
     
   • email of inexisting user
     
   • user already in group
     

NOTE:

• front-end should adapt to new simplified error model in the enveloped response proposed in WIP: 🎨 Enh/error handler 500 #5487
• messages will be more user-friendly after Dump and review automatically all user messages (info, warnings, errors) following Z43 standards #6673
• Mapping between backend exceptions and web-API errors (responses) done in 🎨 web-server: exception handling framework #6655