create: Return error instead of panic when page not found

create: Return error instead of panic when page not found

When `hugo new` creates a content file on a case-insensitive filesystem
where a file with the same name but different case already exists (e.g.
Floop.md vs floop.md), the internal page lookup fails because it uses
exact string comparison. This previously caused an unhelpful panic.

Replace the panic with a descriptive error message that tells the user
what went wrong and suggests checking for case-variant filenames.

Fixes #14112

Co-authored-by: easonysliu <easonysliu@tencent.com>
Co-authored-by: Bjørn Erik Pedersen <bjorn.erik.pedersen@gmail.com>