add warnings for empty groups and sections

src/group.rs

@@ -79,9 +79,18 @@ impl Group {
         let mut sections = HashSet::new();
 
         while lines.peek().is_some() {
-            if let Ok(section) = Section::try_from_lines(&mut lines).context("reading section") {
+            match Section::try_from_lines(&mut lines).context("reading section") {
+                Ok(section) => {
                     sections.insert(section);
                 }
+                Err(e) => {
+                    println!("WARNING: could not process a section under group '{name}': {e:?}\n")
+                }
+            }
+        }
+
+        if sections.is_empty() {
+            println!("WARNING: no sections found in group '{name}'");
         }
 
         Ok(Self { name, sections })

src/section.rs

@@ -5,7 +5,7 @@ use std::{
     iter::Peekable,
 };
 
-use anyhow::{Context, Result};
+use anyhow::{ensure, Context, Result};
 
 use crate::Package;
 
@@ -38,6 +38,9 @@ impl Section {
                 packages.insert(package);
             }
         }
+
+        ensure!(!packages.is_empty());
+
         Ok(Self::new(name, packages))
     }
 }