Stop printing on stderr when adding first coauthor

Before this change, when running `git add-coauthor` without an existing
`~/.git-coauthors` file, the program would print an error message on
stderr that the file did not exist.

It would then create the file with the newly added co-author in it.

That message seemed unnecessary and so it has now been removed.

Co-authored-by: Maria Hansson <mariahansson92@hotmail.com>

src/bin/git-add-coauthor.rs

@@ -1,4 +1,4 @@
-use git_mob::{get_available_coauthors, write_coauthors_file, Author};
+use git_mob::{parse_coauthors_file, write_coauthors_file, Author};
 use structopt::StructOpt;
 
 #[derive(StructOpt, Debug)]
@@ -13,11 +13,12 @@ struct Opt {
 
 fn main() {
     let opt = Opt::from_args();
-    let mut authors = get_available_coauthors();
+    let mut authors = parse_coauthors_file().unwrap_or_default();
     let new_author = Author {
         name: opt.name,
         email: opt.email,
     };
+
     authors.insert(opt.initials, new_author);
 
     write_coauthors_file(authors);

src/lib.rs

@@ -65,7 +65,7 @@ pub fn get_available_coauthors() -> BTreeMap<String, Author> {
     }
 }
 
-fn parse_coauthors_file() -> Result<BTreeMap<String, Author>, Box<dyn Error>> {
+pub fn parse_coauthors_file() -> Result<BTreeMap<String, Author>, Box<dyn Error>> {
     let coauthors_path = coauthors_file_path();
     let coauthors_file = File::open(coauthors_path)?;
     let reader = BufReader::new(coauthors_file);