Standardise `std::cerr` Statements

[nihalzp]

@adisidev: We should change this to follow the style of the rest of our std::cerr statements.

std:cerr << "Error: Hole detected outside polygon! "
  << "ABC"
  << "XYZ"

[adisidev]

This would be a good issue for Amish, and I believe it would give an insightful look into our code.
@atima could you please clarify what format you want?

[atima]

Currently, cartogram-web process stderr line-by-line, searching for: Progress: (.+) and ERROR: (.+).

Please ensure all error messages are in consistent format (case-sensitive) and user-friendly.

For example, Outer boundary is not simple for GeoDiv Mississippi. is_simple() called from project_with_delaunay_t. is too technical (mentions internal functions like is_simple()). It also doesn’t explain the issue or how to fix it.

A good error message should be:

• Clear – Avoid technical jargon. Example:
  ❌ "Error 404: Resource not found."
  ✅ "The page you’re looking for doesn’t exist. Check the URL or go back home."
• Helpful – Explain how to fix the issue. Example:
  ❌ "Invalid input."
  ✅ "Please enter a valid email address (e.g., name@example.com)."
• Polite – Avoid blaming the user. Example:
  ❌ "You entered the wrong password!"
  ✅ "Incorrect password. Try again or reset it here."

If you want some output for debugging, perhaps you can introduce new tag like Debug:.

[adisidev]

Thanks! @amishn2008 this could be good to look into.