changelog: report through the log instead of printing to stdout

generate_entry's status messages become log lines, and the unmet
build-dependency diagnostics travel inside the UnmetBuildDependencies
error (its Display carries the full report) instead of being printed
to stderr by the library: the caller renders both like any other
outcome.

src/debian/deps.rs

@@ -890,7 +890,11 @@ pub struct UnmetBuildDependencies(pub UnmetReport);
 
 impl std::fmt::Display for UnmetBuildDependencies {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "build dependencies/conflicts unsatisfied; aborting")
+        write!(
+            f,
+            "build dependencies/conflicts unsatisfied; aborting\n{}",
+            self.0.message()
+        )
     }
 }