melib/error: do not discard old summary in set_summary

melib/src/error.rs

@@ -92,7 +92,11 @@ impl MeliError {
     where
         M: Into<Cow<'static, str>>,
     {
-        self.summary = Some(summary.into());
+        if let Some(old_summary) = self.summary.take() {
+            self.summary = Some(format!("{}. {}", old_summary, summary.into()).into());
+        } else {
+            self.summary = Some(summary.into());
+        }
         self
     }
 
@@ -118,12 +122,6 @@ impl fmt::Display for MeliError {
     }
 }
 
-impl Into<String> for MeliError {
-    fn into(self) -> String {
-        self.details.into()
-    }
-}
-
 impl Error for MeliError {
     fn source(&self) -> Option<&(dyn Error + 'static)> {
         self.source.as_ref().map(|s| &(*(*s)) as _)