Grammar corrections followup to #510 (#512)

A very small commit, but let's do all the string corrections while I am already at it :-)

Wraps up #510

src/mail.rs

@@ -86,7 +86,7 @@ pub fn build_mail(dest: String, subject: String, body: String) -> Option<Email>
                         env::var("MAIL_SERVER")?
                     )) as Result<_, env::VarError>
                 })
-                .expect("Mail server is not correctly configured"),
+                .expect("The email server is not configured correctly"),
         )
         .to(dest)
         .subject(subject)

src/main.rs

@@ -98,8 +98,8 @@ Then try to restart Plume.
             SearcherError::IndexOpeningError => panic!(
                 r#"
 Plume was unable to open the search index. If you created the index
-before, make sure to run Plume in the directory it was created, or 
-to set SEARCH_INDEX accordingly. If you did not create the search 
+before, make sure to run Plume in the same directory it was created in, or 
+to set SEARCH_INDEX accordingly. If you did not yet create the search 
 index, run this command:
 
     plm search init

src/routes/session.rs

@@ -59,7 +59,7 @@ pub fn create(
     let user_id = if let Ok(user) = user {
         if !user.auth(&form.password) {
             let mut err = ValidationError::new("invalid_login");
-            err.message = Some(Cow::from("Invalid username or password"));
+            err.message = Some(Cow::from("Invalid username, or password"));
             errors.add("email_or_name", err);
             String::new()
         } else {