Allow more detailed explanations behind "Content publish failed"

Hello, recent days arrived alot of people to [this site](http://127.0.0.1:43110/1DdPHedr5Tz55EtQWxqvsbEXPdc4uCVi9D/). But admin removed default site identity and allowed only zeroid.bit which causing all users of the old, site identity seeing "Content publish failed" error which does not help them know how they can fix the issue.

@HelloZeroNet please consider showing more detailed error, like:
"Content publish failed. Zeronet could not find any peers.
Try to advertise your site to get more peers."
"Content publish failed. [what is the cause](linkforthedetailsOfThisParticularCase)"
"Content publish failed. This identity is not allowed, [[Change identity](link)]"

Or Zeronet on page load say "Zeronet detected that you are using unsupported identity on this site. Please click HERE to change identity provider."

Thank you

The issue does not name a file, test, or entry point. Search for the "Content publish failed" message and trace the publishing error cases; done means users receive a specific, actionable explanation for the failure, including unsupported identities and missing peers.

由索引模型根据 Issue 内容生成。