from_file vs from_str ect

it may potentially be useful to introduce a source_error::Errors type which captures multiple errors in one type for the same file

let mut homework_errors = source_error::Errors::new(file, src.lines());
errors.add(
  "made a mistake here", (5, 6)
);
errors.add(
 "made another mistake here", (9, 4)
);
println!("{}", errors);

Display is for humans but rust's default termination behavior for main methods is to render Debug. Hence you might expect this to pretty print the way Display does but instead you'll get the render of the Error types struct fields.

use std::error::Error;
fn main() -> Result<(), Box<dyn Error>> {
   Err(source_error::from_file(...))?;
   Ok(())
}

perhaps underscore the letter at that location, but that wont play well when colors are disabled. ^ is typical but may make the line spacing weird

tuple works but doesn't extract much