Template picker does not handle file name conflicts

[juliushaertl]

Steps to reproduce

2. Already have a "New document.odt" in the current directory
3. Create a new file from a template
4. See "New document.odt" as the file name
5. Continue to create that file
6. See an error "Unable to create new file from template"

How it should behave

• Ideally we show that there is a naming conflict already when entering the name
• Create a new unique filename by appending a number e.g. "New document (2).odt" as a predefined name
• Show a more detailed error message as the server already responds with:

@elzody Maybe you can check that when looking into the template picker for nextcloud/richdocuments#3852