Implement Error Handling to Inventor's Assistant

[Catrovitch]

• Error handling when calling LLM. Failed call should be handled, communicated and user should be able to try again.
• [x] User can not submit while user input fields are empty
• [x] After submitting a user cannot change the input in the input field. This is also visually indicated by greying out the input field.
• [x] If a call to the backend fails there is handling of the Errors
• [x] A general Timeout of 15 sec is used for UserInputValidation
• [ ] When an error occurs it is communicated to the User
• [x] The user can try again if an error occurs