Error messages are part of the interface

#Name the failed operation

“Invalid input” says almost nothing. “Cannot read notes/index.md” identifies both the attempted operation and its target. The underlying operating-system error can then explain why it failed.

#Context should earn its place

Repeatedly wrapping the same error makes a message longer without making it more useful. Add context at the boundary that knows the concrete operation; let clear errors pass through intermediate layers unchanged.

let source = fs::read_to_string(path)    .with_context(|| format!("cannot read {}", path.display()))?;